Add support to xenconfig for conversion of xl.cfg(5) bios config
to/from libvirt domXml <loader> config. SeaBIOS is the default
for HVM guests using upstream QEMU. ROMBIOS is the default when
using the old qemu-dm. This patch allows specifying OVMF as an
alternate firmware.

Example xl.cfg:
  bios = "ovmf"

Example domXML:
  <os>
    ...
    <loader readonly='yes' type='pflash'>/usr/lib/xen/boot/ovmf.bin</loader>
  </os>

Note that currently Xen does not support a separate nvram for
non-volatile variables.
Signed-off-by: NJim Fehlig <jfehlig@suse.com>