vpci/msix: add MSI-X handlers

vpci/msix: add MSI-X handlers

Add handlers for accesses to the MSI-X message control field on the
PCI configuration space, and traps for accesses to the memory region
that contains the MSI-X table and PBA. This traps detect attempts from
the guest to configure MSI-X interrupts and properly sets them up.

Note that accesses to the Table Offset, Table BIR, PBA Offset and PBA
BIR are not trapped by Xen at the moment.

Finally, turn the panic in the Dom0 PVH builder into a warning.

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>