This capability tracks whether QEMU supports -fw_cfg command line
option, more specifically whether it allows specifying filename.

There are some releases of QEMU which support -fw_cfg but not
filename. If this is ever a problem we can refine the capability
later on.
Signed-off-by: NMichal Privoznik <mprivozn@redhat.com>
Reviewed-by: NDaniel P. Berrangé <berrange@redhat.com>