Description of problem:

In Compute -> Virtual Machines -> RHEL8 -> Disks -> Logical Name, the user expects to see the path to the disk device inside the VM (i.e. /dev/sda).
This comes from VDSM guestDiskMapping.

When ovirt-guest-agent is not present (RHEL8) this info comes from qemu-guest-agent.
However, VDSM seems to be using the information from "guest-get-fsinfo", which returns mounted filesystems in the Guest, which can be problematic.

1. See this RHEL8 Guest, with 2 disks and only 1 disk has a fs mounted, its missing an entry for the second disk.

    "guestDiskMapping": {
        "0QEMU_QEMU_HARDDISK_af3c6af3-0154-4319-b": {
            "name": "/dev/sda2"
        }
    }, 

The user sees this, there is no mapping for the second disk.

engine=# select device_id,logical_name from vm_device_view where device='disk' and vm_id = '7606d359-b199-483c-8c05-92a4392b970a';
              device_id               | logical_name 
--------------------------------------+--------------
 8abc438a-46a4-4961-bd78-ad6ab376e526 | 
 af3c6af3-0154-4319-b368-0f64d24d7d72 | /dev/sda2

2. Now I create 4 partitions on the second disk (sdb), a filesystem on sdb4 and mount it:

VDSM reports:

    "guestDiskMapping": {
        "0QEMU_QEMU_HARDDISK_af3c6af3-0154-4319-b": {
            "name": "/dev/sda2"
        }, 
        "0QEMU_QEMU_HARDDISK_8abc438a-46a4-4961-b": {
            "name": "/dev/sdb4"
        }
    }, 

The user sees:

engine=# select device_id,logical_name from vm_device_view where device='disk' and vm_id = '7606d359-b199-483c-8c05-92a4392b970a';
              device_id               | logical_name 
--------------------------------------+--------------
 8abc438a-46a4-4961-bd78-ad6ab376e526 | /dev/sdb4
 af3c6af3-0154-4319-b368-0f64d24d7d72 | /dev/sda2

Both are wrong, it should be /dev/sdb and /dev/sda.

3. Add a third disk, with LVM, missing again:

engine=# select device_id,logical_name from vm_device_view where device='disk' and vm_id = '7606d359-b199-483c-8c05-92a4392b970a';
              device_id               | logical_name 
--------------------------------------+--------------
 8abc438a-46a4-4961-bd78-ad6ab376e526 | /dev/sdb4
 af3c6af3-0154-4319-b368-0f64d24d7d72 | /dev/sda2
 b603caa2-6b39-4e52-9182-a915e1764186 | 

4. Create a filesystem on the LVM and mount it. Finally it shows up, and its the only correct one this time.

engine=# select device_id,logical_name from vm_device_view where device='disk' and vm_id = '7606d359-b199-483c-8c05-92a4392b970a';
              device_id               | logical_name 
--------------------------------------+--------------
 8abc438a-46a4-4961-bd78-ad6ab376e526 | /dev/sdb4
 af3c6af3-0154-4319-b368-0f64d24d7d72 | /dev/sda2
 b603caa2-6b39-4e52-9182-a915e1764186 | /dev/sdc

So the problems are:
* No mapping if file-system not mounted
* The mapping can be to a partition instead of the device

Version-Release number of selected component (if applicable):
vdsm-4.30.38-1.el7ev.x86_64

How reproducible:
Always

Steps to Reproduce:
1. Install RHEL8 Guest
2. Notice that 'Logical Name' in VM Disks tab comes from mounted filesystem device paths, not the disks.
3. If file-system not mounted, there is no Logical Name reported.

Actual results:
* Missing or wrong Logical Names reported

Expected results:
* Correct Logical Names for disks

Additional info:
For reference, for the example I gave above.

A) Inside the Guest
/dev/mapper/rhel-root on / type xfs (rw,relatime,seclabel,attr2,inode64,noquota)
/dev/sda1 on /boot type xfs (rw,relatime,seclabel,attr2,inode64,noquota)
/dev/sdb4 on /mnt type xfs (rw,relatime,seclabel,attr2,inode64,noquota)
/dev/mapper/germano_vg-lvol0 on /mnt2 type xfs (rw,relatime,seclabel,attr2,inode64,noquota)

B) vdsm-client VM getInfo vmID=7606d359-b199-483c-8c05-92a4392b970a
    "guestDiskMapping": {
        "0QEMU_QEMU_HARDDISK_af3c6af3-0154-4319-b": {
            "name": "/dev/sda2"
        }, 
        "0QEMU_QEMU_HARDDISK_b603caa2-6b39-4e52-9": {
            "name": "/dev/sdc"
        }, 
        "0QEMU_QEMU_HARDDISK_8abc438a-46a4-4961-b": {
            "name": "/dev/sdb4"
        }


C) virsh qemu-agent-command RHEL8 '{"execute":"guest-get-fsinfo"}'
{
    "return": [
        {
            "disk": [
                {
                    "bus": 0,
                    "bus-type": "scsi",
                    "dev": "/dev/sdc",
                    "pci-controller": {
                        "bus": 0,
                        "domain": 0,
                        "function": 0,
                        "slot": 5
                    },
                    "serial": "0QEMU_QEMU_HARDDISK_b603caa2-6b39-4e52-9",
                    "target": 0,
                    "unit": 3
                }
            ],
            "mountpoint": "/mnt2",
            "name": "dm-2",
            "type": "xfs"
        },
        {
            "disk": [
                {
                    "bus": 0,
                    "bus-type": "scsi",
                    "dev": "/dev/sdb4",
                    "pci-controller": {
                        "bus": 0,
                        "domain": 0,
                        "function": 0,
                        "slot": 5
                    },
                    "serial": "0QEMU_QEMU_HARDDISK_8abc438a-46a4-4961-b",
                    "target": 0,
                    "unit": 2
                }
            ],
            "mountpoint": "/mnt",
            "name": "sdb4",
            "type": "xfs"
        },
        {
            "disk": [
                {
                    "bus": 0,
                    "bus-type": "scsi",
                    "dev": "/dev/sda1",
                    "pci-controller": {
                        "bus": 0,
                        "domain": 0,
                        "function": 0,
                        "slot": 5
                    },
                    "serial": "0QEMU_QEMU_HARDDISK_af3c6af3-0154-4319-b",
                    "target": 0,
                    "unit": 0
                }
            ],
            "mountpoint": "/boot",
            "name": "sda1",
            "type": "xfs"
        },
        {
            "disk": [
                {
                    "bus": 0,
                    "bus-type": "scsi",
                    "dev": "/dev/sda2",
                    "pci-controller": {
                        "bus": 0,
                        "domain": 0,
                        "function": 0,
                        "slot": 5
                    },
                    "serial": "0QEMU_QEMU_HARDDISK_af3c6af3-0154-4319-b",
                    "target": 0,
                    "unit": 0
                }
            ],
            "mountpoint": "/",
            "name": "dm-0",
            "type": "xfs"
        }
    ]
}