snapshot: detect when qemu lacks disk-snapshot support

snapshot: detect when qemu lacks disk-snapshot support

Noticed when testing new libvirt against old qemu that lacked the
snapshot_blkdev HMP command.  Libvirt was mistakenly treating the
command as successful, and re-writing the domain XML to use the
just-created 0-byte file, rendering the domain broken on restart.

* src/qemu/qemu_monitor_text.c (qemuMonitorTextDiskSnapshot):
Notice another possible error message.
* src/qemu/qemu_driver.c
(qemuDomainSnapshotCreateSingleDiskActive): Don't keep 0-byte file
on failure.