Populate libxl_domain_build_info struct with bios and firmware
info from virDomainLoaderDef.

Note: Currently libxl only allows specifying the type of BIOS.
For type LIBXL_BIOS_TYPE_OVMF, the firmware path is configured
when building Xen using '--with-system-ovmf='. If not specified,
LIBXL_FIRMWARE_DIR/ovmf.bin is used. In the future, Xen will
support a user-specified firmware path. See

http://lists.xenproject.org/archives/html/xen-devel/2016-03/msg01628.html

Once that work is merged into xen.git, the libvirt libxl driver
will be able to honor a user-specified path. In the meantime use
the implicit path, which is tolerable since it is advertised in
domcapabilities.
Signed-off-by: NJim Fehlig <jfehlig@suse.com>