qemu: Filter firmware images by type

qemu: Filter firmware images by type

If the configuration explicitly requests a specific type of
firmware image, be it pflash or ROM, we should ignore all images
that are not of that type.

If no specific type has been requested, of course, any type is
considered a match and the selection will be based upon the
other attributes.

Signed-off-by: Andrea Bolognani <abologna@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>