libxl: populate build_info vfb in separate function

libxl: populate build_info vfb in separate function

For HVM domains, vfb info must be populated in the libxl_domain_build_info
struct.  Currently this is done in the libxlMakeVfbList function, but IMO
it would be cleaner to populate the build_info vfb in a separate
libxlMakeBuildInfoVfb function.  libxlMakeVfbList would then handle only
vfb devices, simiar to the other libxlMake<device>List functions.

A future patch will extend libxlMakeBuildInfoVfb to support SPICE.

Signed-off-by: Jim Fehlig <jfehlig@suse.com>