vfio_pci: Allow regions to add own capabilities

VFIO regions already support region capabilities with a limited set of
fields. However the subdriver might have to report to the userspace
additional bits.

This adds an add_capability() hook to vfio_pci_regops.

Signed-off-by: Alexey Kardashevskiy <aik@ozlabs.ru>
Acked-by: Alex Williamson <alex.williamson@redhat.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>