hyperv-fb: add support for generation 2 virtual machines.

UEFI-based generation 2 virtual machines support vmbus devices only.
There is no pci bus.  Thus they use a different mechanism for the
graphics framebuffer:  Instead of using the vga pci bar a chunk of
memory muct be allocated from the hyperv mmio region declared using
APCI.  This patch implements support for it.

Based on a patch by Haiyang Zhang <haiyangz@microsoft.com>

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Signed-off-by: Haiyang Zhang <haiyangz@microsoft.com>
Acked-by: Tomi Valkeinen <tomi.valkeinen@ti.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>