qemu: Clear os.firmwareFeatures after autoselection

qemu: Clear os.firmwareFeatures after autoselection

We already clear os.firmware, so it doesn't make sense to keep
the list of features around.

Moreover, our validation routines will reject an XML that
contains a list of firmware features but disables firmware
autoselection, so not clearing these means that the live XML
for a domain that uses feature-based autoselection can't be
fed back into libvirt.

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