xen/vga: add the xen EFI video mode support

In order to add xen EFI frambebuffer video support, it is required to add
xen-efi's new video type (XEN_VGATYPE_EFI_LFB) case and handle it in the
function xen_init_vga and set the video type to VIDEO_TYPE_EFI to enable
efi video mode.

The original patch from which this was broken out from:
 http://marc.info/?i=4E099AA6020000780004A4C6@nat28.tlf.novell.com

Signed-off-by: Jan Beulich <JBeulich@novell.com>
Signed-off-by: Tang Liang <liang.tang@oracle.com>
[v2: The original author is Jan Beulich and Liang Tang ported it to upstream]
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>