qemu: mkdir memory_backing_dir on startup
    
    In 48d9e6cdcc 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: Michal Privoznik <mprivozn@redhat.com>