qemu: Use predictable file names for memory-backend-file

qemu: Use predictable file names for memory-backend-file

In some cases management application needs to allocate memory for
qemu upfront and then just let qemu use that. Since we don't want
to expose path for memory-backend-file anywhere in the domain
XML, we can generate predictable paths. In this case:

  $memoryBackingDir/libvirt/qemu/$shortName/$alias

where $shortName is result of virDomainDefGetShortName().

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: John Ferlan <jferlan@redhat.com>