vpci: add a priority parameter to the vPCI register initializer

vpci: add a priority parameter to the vPCI register initializer

This is needed for MSI-X, since MSI-X will need to be initialized
before parsing the BARs, so that the header BAR handlers are aware of
the MSI-X related holes and make sure they are not mapped in order for
the trap handlers to work properly.

Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
[ARM]
Acked-by: Julien Grall <julien.grall@arm.com>