libxl: fix framebuffer port setting for HVM domains

libxl: fix framebuffer port setting for HVM domains

libxl uses the libxl_vnc_info and libxl_sdl_info fields from the
hvm union in libxl_domain_build_info struct when generating QEMU
args for VNC or SDL.  These fields were left unset by the libxl
driver, causing libxl to ignore any user settings.  E.g. with

  <graphics type='vnc' port='5950'/>

port would be ignored and QEMU would instead be invoked with

  -vnc 127.0.0.1:0,to=99

Unlike the libxl_domain_config struct, the libxl_domain_build_info
contains only a single libxl_vnc_info and libxl_sdl_info, so
populate these fields from the first vfb in
libxl_domain_config->vfbs.

Signed-off-by: Jim Fehlig <jfehlig@suse.com>
Signed-off-by: David Kiarie <davidkiarie4@gmail.com>