Description of problem:
libvirt versions prior to 1.2.2 do not handle type='block' properly, and require VDSM to send the legacy file=path syntax too. Since we now (checked on commit 04f610b644d605fe9fb6f7ecc2a1178fbf30668d) require libvirt 1.2.8 (at least, depending on the platform), this workaround in vm.py is redundant, and should be removed.

The relevant piece of code, in vm.py circa line 3066:

       def _diskSnapshot(vmDev, newPath, sourceType):
            """Libvirt snapshot XML"""

            disk = vmxml.Element('disk', name=vmDev, snapshot='external',
                                 type=sourceType)
            # Libvirt versions before 1.2.2 do not understand 'type' and treat
            # all snapshots as if they are type='file'.  In order to ensure
            # proper handling of block snapshots in modern libvirt versions,
            # we specify type='block' and dev=path for block volumes but we
            # always speficy the file=path for backwards compatibility.
            args = {'type': sourceType, 'file': newPath}
            if sourceType == 'block':
                args['dev'] = newPath
            disk.appendChildWithArgs('source', **args)
            return disk