In 48d9e6cd and friends we've allowed users to back guest
memory by a file inside the host. And in order to keep things
manageable the memory_backing_dir variable was introduced to
qemu.conf to specify the directory where the files are kept.
However, libvirt's policy is that directories are created on
domain startup if they don't exist. We've missed this one.
Signed-off-by: NMichal Privoznik <mprivozn@redhat.com>