x86: enable multi-vector MSI

x86: enable multi-vector MSI

This implies
- extending the public interface to have a way to request a block of
  MSIs
- allocating a block of contiguous pIRQ-s for the target domain (but
  note that the Xen IRQs allocated have no need of being contiguous)
- repeating certain operations for all involved IRQs
- fixing multi_msi_enable()
- adjusting the mask bit accesses for maskable MSIs

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: Keir Fraser <keir@xen.org>