qemu_firmware: don't error out for unknown firmware features

qemu_firmware: don't error out for unknown firmware features

When QEMU introduces new firmware features libvirt will fail until we
list that feature in our code as well which doesn't sound right.

We should simply ignore the new feature until we add a proper support
for it.

Reported-by: Laszlo Ersek <lersek@redhat.com>
Signed-off-by: Pavel Hrdina <phrdina@redhat.com>
Reviewed-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>