Description of problem:
-----------------------
According to the manpage it should be possible to restrict the disks being snapshoted:

  "Normally, the only properties settable for a domain snapshot are the <name>
  and <description> elements, as well as <disks> if --disk-only is given; the
  rest of the fields are ignored, and automatically filled in by libvirt."

However virsh seems to ignore the <disks> element and always snapshot all disks.


Version-Release number of selected component (if applicable):
-------------------------------------------------------------
libvirt 1.3.0


How reproducible:
-----------------
See below


Steps to Reproduce:
-------------------
1. Create a temporary directory:
   mkdir /tmp/test

2. Create a storage pool:
   virsh -c qemu:///session pool-create-as testpool dir --target /tmp/test

3. Create two qcow2 disk images:
   virsh -c qemu:///session vol-create-as --format qcow2 testpool Test1 1G
   virsh -c qemu:///session vol-create-as --format qcow2 testpool Test2 1G

4. Create a domain using these images:
   virsh -c qemu:///session define /dev/stdin <<EOF
<domain type='kvm'>
  <name>Test</name>
  <uuid>f44e2d21-32be-4129-936f-440e75c691a5</uuid>
  <memory unit='KiB'>1048576</memory>
  <currentMemory unit='KiB'>1048576</currentMemory>
  <vcpu placement='static'>1</vcpu>
  <os>
    <type arch='x86_64' machine='pc-i440fx-2.4'>hvm</type>
  </os>
  <devices>
    <emulator>/usr/bin/qemu-system-x86_64</emulator>
    <disk type='file' device='disk'>
      <driver name='qemu' type='qcow2'/>
      <source file='/tmp/test/Test1'/>
      <target dev='hda' bus='ide'/>
      <address type='drive' controller='0' bus='0' target='0' unit='0'/>
    </disk>
    <disk type='file' device='disk'>
      <driver name='qemu' type='qcow2'/>
      <source file='/tmp/test/Test2'/>
      <target dev='hdb' bus='ide'/>
      <address type='drive' controller='0' bus='0' target='0' unit='1'/>
    </disk>
  </devices>
</domain>
EOF

5. Inspect the contents of the storage pool BEFORE taking the snapshot:
   ls -la /tmp/test

6. Create a snapshot:
   virsh -c qemu:///session snapshot-create Test --disk-only /dev/stdin <<EOF
<domainsnapshot>
  <name>Snapshot1</name>
  <disks>
    <disk name="/tmp/test/Test1" snapshot="external">
      <source file="/tmp/test/Test1_Snapshot" />
    </disk>
  </disks>
</domainsnapshot>
EOF

7. Inspect the contents of the storage pool AFTER taking the snapshot:
   ls -la /tmp/test


Actual results:
---------------
Before taking the snapshot:

drwxr-xr-x  2 pief users   4096 19. Jan 13:49 .
drwxrwxrwt 72 root root   28672 19. Jan 13:49 ..
-rw-------  1 pief users 197120 19. Jan 13:49 Test1
-rw-------  1 pief users 197120 19. Jan 13:49 Test2

After taking the snapshot:

drwxr-xr-x  2 pief users   4096 19. Jan 13:49 .
drwxrwxrwt 72 root root   28672 19. Jan 13:49 ..
-rw-------  1 pief users 197120 19. Jan 13:49 Test1
-rw-r--r--  1 pief users 197120 19. Jan 13:49 Test1_Snapshot
-rw-------  1 pief users 197120 19. Jan 13:49 Test2
-rw-r--r--  1 pief users 197120 19. Jan 13:49 Test2.Snapshot1


Expected results:
-----------------
There shouldn't have been a file "Test2.Snapshot1" because the XML specified that only Test1 should be snapshotted.


Additional info:
----------------
Seeing that it's named "Test2.Snapshot1" and not "Test2_Snapshot", it appears libvirt is not RESTRICTING snapshoting to Test1 but merely CHANGING the snapshot parameters for Test1 while maintaining the default behavior for Test2.
(The default name for a snapshot is constructed by libvirt by appending the snapshot to the domain name, separated by a colon ".")