qemu: Drop two layers of nesting of memoryBackingDir

qemu: Drop two layers of nesting of memoryBackingDir

Initially introduced in v3.10.0-rc1~172.

When generating a path for memory-backend-file or -mem-path, qemu
driver will use the following pattern:

  $memoryBackingDir/libvirt/qemu/$id-$shortName

where $memoryBackingDir defaults to /var/lib/libvirt/qemu/ram but
can be overridden in qemu.conf. Anyway, the "/libvirt/qemu/" part
looks redundant, because it's already contained in the default,
or creates unnecessary nesting if overridden in qemu.conf.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Andrea Bolognani <abologna@redhat.com>