Commit 6766ff10 introduced a corner case bug with snapshot creation:
if a snapshot is created, but then we hit OOM while trying to
create the return value of the function, then we have polluted the
internal directory with the snapshot metadata with no way to clean
it up from the running libvirtd.

* src/qemu/qemu_driver.c (qemuDomainSnapshotCreateXML): Don't
write metadata file on OOM condition.