vgabios update: handle compatibility with older qemu versions

As pointed out by avi the vgabios update is guest-visible and thus has
migration implications.

One change is that the vga has a valid pci rom bar now.  We already have
a pci bus property to enable/disable the rom bar and we'll load the bios
via fw_cfg as fallback for the no-rom-bar case.  So we just have to add
compat properties to handle this case.

A second change is that the magic bochs lfb @ 0xe0000000 is gone.  When
live-migrating a guest from a older qemu version it might be using the
lfb though, so we have to keep it for the old machine types.  The patch
enables the bochs lfb in case we don't have the pci rom bar enabled
(i.e. we are in 0.13+older compat mode).

This patch depends on these patches which add (and use) the pc-0.13
machine type:
  http://patchwork.ozlabs.org/patch/70797/
  http://patchwork.ozlabs.org/patch/70798/

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Cc: avi@redhat.com
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
