vfio/pci: Fix VGA quirks

Commit 2d82f8a3 ("vfio/pci: Convert all MemoryRegion to dynamic
alloc and consistent functions") converted VFIOPCIDevice.vga to be
dynamically allocted, negating the need for VFIOPCIDevice.has_vga.
Unfortunately not all of the has_vga users were converted, nor was
the field removed from the structure.  Correct these oversights.

Reported-by: Peter Maloney <peter.maloney@brockmann-consult.de>
Tested-by: Peter Maloney <peter.maloney@brockmann-consult.de>
Fixes: 2d82f8a3 ("vfio/pci: Convert all MemoryRegion to dynamic alloc and consistent functions")
Fixes: https://bugs.launchpad.net/qemu/+bug/1591628


Cc: qemu-stable@nongnu.org
Signed-off-by: Alex Williamson <alex.williamson@redhat.com>