vpci/msi: add MSI handlers

vpci/msi: add MSI handlers

Add handlers for the MSI control, address, data and mask fields in
order to detect accesses to them and setup the interrupts as requested
by the guest.

Note that the pending register is not trapped, and the guest can
freely read/write to it.

Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
[IO]
Reviewed-by: Paul Durrant <paul.durrant@citrix.com>