vfio/pci: Add base BAR MemoryRegion

vfio/pci: Add base BAR MemoryRegion

Add one more layer to our stack of MemoryRegions, this base region
allows us to register BARs independently of the vfio region or to
extend the size of BARs which do map to a region.  This will be
useful when we want hypervisor defined BARs or sections of BARs,
for purposes such as relocating MSI-X emulation.  We therefore call
msix_init() based on this new base MemoryRegion, while the quirks,
which only modify regions still operate on those sub-MemoryRegions.

Signed-off-by: Alex Williamson <alex.williamson@redhat.com>