qemu: Make firmware parsing failures non-fatal

qemu: Make firmware parsing failures non-fatal

At the moment, any kind of issue being detected in any of the
firmware descriptor files will result in the entire process
being aborted.

In particular, installing a build of edk2 for an architecture
that libvirt doesn't yet know about, for example loongarch64,
will break most firmware-related functionality: it will no
longer be possible to define new EFI VMs, start existing ones,
or even just obtain the domcapabilities for any architecture.

This is obviously unnecessarily harsh. Adopt a more relaxed
approach and simply ignore the firmware descriptors that we
are unable to parse correctly.

https://bugzilla.redhat.com/show_bug.cgi?id=2258946

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