qemu: Fix matching for stateless/combined firmware

qemu: Fix matching for stateless/combined firmware

The current code assumes that a stateless firmware has to be
explicitly requested by the user, and should never be picked
otherwise. This means that, for example, domains configured to
use SEV-SNP are forced to explicitly request for the firmware
to be stateless.

Additionally, we assume that only split firmware is suitable
for the stateful use case, whereas a combined firmware image
would also do the job.

As a result of these changes, the failing SEV-SNP test case
that was added recently passes, and so do the test cases
requesting read/write firmware.

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