A sane guest should not rely on the automatically generated /dev/sdX names to address a partition. Even if the order in a single bus is consistent, attaching a different bus (for example, ide/ahci) could as easily break the names.

Alternatively, there are /dev/disk/by-* symbol links that are more robust to system configuration changes. If you really need persistent /dev/sdX names, use udev persistent device name rules.

Fam

By default, RHEL 6 installation (and almost every Linux distribution) uses UUID to find boot and root filesystem devices, so it will work still after you plug new disks that changes /dev/sdX order.

I had the same issue after switching from virtio-blk to virtio-scsi
udev does not cache sda pci map even if you add serial to the disk.

As workaround, I changed the target (on top of the unit) and 
it is working.

BEFORE

      <address type='drive' controller='0' bus='0' target='0' unit='0'/>
      <address type='drive' controller='0' bus='0' target='0' unit='1'/>
      <address type='drive' controller='0' bus='0' target='0' unit='2'/>
      <address type='drive' controller='0' bus='0' target='0' unit='3'/>

AFTER (workaround)

      <address type='drive' controller='0' bus='0' target='0' unit='0'/>
      <address type='drive' controller='0' bus='0' target='1' unit='1'/>
      <address type='drive' controller='0' bus='0' target='2' unit='2'/>
      <address type='drive' controller='0' bus='0' target='3' unit='3'/>

    <disk type='block' device='disk'>
      <driver name='qemu' type='raw' cache='writeback' discard='unmap'/>
      <source dev='/var/lib/libvirt/images/boot.raw'/>
      <target dev='sda' bus='scsi'/>
      <serial>42-001</serial>
      <boot order='1'/>
      <alias name='scsi0-0-0-0'/>
      <address type='drive' controller='0' bus='0' target='0' unit='0'/>
    </disk>
    <disk type='block' device='disk'>
      <driver name='qemu' type='raw' cache='writeback' discard='unmap'/>
      <source dev='/var/lib/libvirt/images/root.raw'/>
      <target dev='sdb' bus='scsi'/>
      <serial>42-002</serial>
      <alias name='scsi0-0-1-1'/>
      <address type='drive' controller='0' bus='0' target='1' unit='1'/>
    </disk>
    ...
    <controller type='scsi' index='0' model='virtio-scsi'>
      <alias name='scsi0'/>
      <address type='pci' domain='0x0000' bus='0x00' slot='0x05' function='0x0'/>
    </controller>

The interessting part is also that the ID_SCSI does not change?!

for dev in sd{a..d}; do echo -e "\n[$dev]"; udevadm info --query=all --name=/dev/$dev | grep -E '(ID_SCSI|ID_PATH)'; done

[sda]
E: ID_PATH=pci-0000:00:05.0-virtio-pci-virtio2-scsi-0:0:0:0
E: ID_PATH_TAG=pci-0000_00_05_0-virtio-pci-virtio2-scsi-0_0_0_0
E: ID_SCSI=1
E: ID_SCSI_SERIAL=42-001

[sdb]
E: ID_PATH=pci-0000:00:05.0-virtio-pci-virtio2-scsi-0:0:1:1
E: ID_PATH_TAG=pci-0000_00_05_0-virtio-pci-virtio2-scsi-0_0_1_1
E: ID_SCSI=1
E: ID_SCSI_SERIAL=42-002

[sdc]
E: ID_PATH=pci-0000:00:05.0-virtio-pci-virtio2-scsi-0:0:2:2
E: ID_PATH_TAG=pci-0000_00_05_0-virtio-pci-virtio2-scsi-0_0_2_2
E: ID_SCSI=1
E: ID_SCSI_SERIAL=42-003

[sdd]
E: ID_PATH=pci-0000:00:05.0-virtio-pci-virtio2-scsi-0:0:3:3
E: ID_PATH_TAG=pci-0000_00_05_0-virtio-pci-virtio2-scsi-0_0_3_3
E: ID_SCSI=1
E: ID_SCSI_SERIAL=42-004

How do the possible need to modify a change?

Virtio-scsi must either modify the config volume may seem normal?