qemu: introduce vram64 attribute for QXL video device

qemu: introduce vram64 attribute for QXL video device

This attribute is used to extend secondary PCI bar and expose it to the
guest as 64bit memory.  It works like this: attribute vram is there to
set size of secondary PCI bar and guest sees it as 32bit memory,
attribute vram64 can extend this secondary PCI bar.  If both attributes
are used, guest sees two memory bars, both address the same memory, with
the difference that the 32bit bar can address only the first part of the
whole memory.

Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=1260749

Signed-off-by: Pavel Hrdina <phrdina@redhat.com>