qemu: Fix matching for read/write firmware

qemu: Fix matching for read/write firmware

We currently always pick a read-only firmware unless we are
explicitly asked for a read/write one, which is probably what
most people expect anyway but doesn't really make sense
otherwise: if no specific requirement has been provided by the
user, both read-only and read/write firmwares should be
allowed to match.

This won't result in any change in practice, since distros are
not shipping read/write builds of edk2 anyway. If they started
doing that, it would be their responsibility to ensure that
they are ordered after the read-only builds.

Signed-off-by: Andrea Bolognani <abologna@redhat.com>
Reviewed-by: Jim Fehlig <jfehlig@suse.com>