qemu: Filter firmwares based on format

qemu: Filter firmwares based on format

If the user has requested a specific firmware format, then
all firmware builds that are not in that format should be
ignored while looking for matches.

The legacy hardcoded firmware list predates firmware
descriptors and their "format" field, so we can safely
assume that all builds listed in there are in raw format.

Signed-off-by: Andrea Bolognani <abologna@redhat.com>
Reviewed-by: Michal Privoznik <mprivozn@redhat.com>