conf: Introduce firmware attribute to <os/>

conf: Introduce firmware attribute to <os/>

The idea is that using this attribute users enable libvirt to
automagically select firmware image for their domain. For
instance:

  <os firmware='efi'>
    <type arch='x86_64' machine='pc-q35-4.0'>hvm</type>
    <loader secure='no'/>
  </os>

  <os firmware='bios'>
    <type arch='x86_64' machine='pc-q35-4.0'>hvm</type>
  </os>

(The automagic of selecting firmware image will be described in
later commits.)

Accepted values are 'bios' and 'efi' to let libvirt select
corresponding type of firmware.

I know it is a good sign to introduce xml2xml test case when
changing XML config parser but that will have to come later.
Firmware auto selection is not enabled for any driver just yet so
any xml2xml test would fail right away.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>