vbe: make bochs dispi interface return the correct memory size with qxl

VgaState->vram_size is the size of the pci bar.  In case of qxl not the
whole pci bar can be used as vga framebuffer.  Add a new variable
vbe_size to handle that case.  By default (if unset) it equals
vram_size, but qxl can set vbe_size to something else.

This makes sure VBE_DISPI_INDEX_VIDEO_MEMORY_64K returns correct results
and sanity checks are done with the correct size too.

Cc: qemu-stable@nongnu.org
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>
