Bug 1127226

Description of problem:
Currently there's no sane way (except for using --reuse-external) to specify a relative backing name to be stored in the newly created snapshot and passing a relative file name doesn't work.

+++ This bug was initially created as a clone of Bug #1126329 +++

Version-Release number of selected component (if applicable):

libvirt-1.1.1-29.el7_0.1.x86_64


How reproducible:
100%

Steps to Reproduce:

1. Create external disk snapshot with relative path

# pwd
/root

# virsh snapshot-create-as rhel6 s1 --diskspec vda,file=./rhel6.s1 --disk-only
Domain snapshot s1 created

# virsh snapshot-dumpxml rhel6 s1

<domainsnapshot>
  <name>s1</name>
  <state>disk-snapshot</state>
  <creationTime>1406813388</creationTime>
  <memory snapshot='no'/>
  <disks>
    <disk name='vda' snapshot='external' type='file'>
      <driver type='qcow2'/>
      <source file='./rhel6.s1'/>
    </disk>
  </disks>

# virsh dumpxml rhel6
..
<disk type='file' device='disk'>
      <driver type='qcow2' cache='none'/>
      <source file='./rhel6.s1'/>
      <backingStore type='file' index='1'>
        <format type='qcow2'/>
        <source file='/usr/local/var/lib/libvirt/images/rhel6.img'/>
        <backingStore/>
      </backingStore>
      <target dev='vda' bus='virtio'/>
      <alias name='virtio-disk0'/>
      <address type='pci' domain='0x0000' bus='0x00' slot='0x06' function='0x0'/>
    </disk>
..

2. Create external disk snapshot with absolute path

# virsh snapshot-create-as rhel6 s2 --diskspec vda,file=/usr/local/var/lib/libvirt/images/rhel6.s2 --disk-only
Domain snapshot s2 created

# virsh dumpxml rhel6

..
 <disk type='file' device='disk'>
      <driver type='qcow2' cache='none'/>
      <source file='/usr/local/var/lib/libvirt/images/rhel6.s2'/>
      <target dev='vda' bus='virtio'/>
      <alias name='virtio-disk0'/>
      <address type='pci' domain='0x0000' bus='0x00' slot='0x06' function='0x0'/>
    </disk>
..

#qemu-img  info /usr/local/var/lib/libvirt/images/rhel6.s2
image: /usr/local/var/lib/libvirt/images/rhel6.s2
file format: qcow2
virtual size: 3.9G (4194304000 bytes)
disk size: 712K
cluster_size: 65536
backing file: ./rhel6.s1 (actual path: /usr/local/var/lib/libvirt/images/./rhel6.s1)


3. Create external disk snapshot 

# virsh snapshot-create-as rhel6 s3  --disk-only
error: internal error: unable to execute QEMU command 'transaction': Could not open '/usr/local/var/lib/libvirt/images/rhel6.s2': Could not open file: No such file or directory: No such file or directory


# ll /usr/local/var/lib/libvirt/images/rhel6.s2
-rw-------. 1 root root 2228224 Jul 31 21:42 /usr/local/var/lib/libvirt/images/rhel6.s2 

Actual results:


Expected results:


Additional info: