vfio: unmap and free BAR data in instance_finalize

In the case of VFIO, the unrealize callback is too early to munmap the
BARs.  The munmap must be delayed until memory accesses are complete.
To do this, split vfio_unmap_bars in two.  The removal step, now called
vfio_unregister_bars, remains in vfio_exitfn.  The reclamation step
is vfio_unmap_bars and is moved to the instance_finalize callback.

Similarly, quirk MemoryRegions have to be removed during
vfio_unregister_bars, but freeing the data structure must be delayed
to vfio_unmap_bars.

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