viridian: add ExProcessorMasks variant of the IPI hypercall

viridian: add ExProcessorMasks variant of the IPI hypercall

A previous patch introduced variants of the flush hypercalls that take a
'Virtual Processor Set' as an argument rather than a simple 64-bit mask.
This patch introduces a similar variant of the HVCALL_SEND_IPI hypercall
(HVCALL_SEND_IPI_EX).

NOTE: As with HVCALL_FLUSH_VIRTUAL_ADDRESS_SPACE/LIST_EX, a guest should
      not yet issue the HVCALL_SEND_IPI_EX hypercall as support for
      'ExProcessorMasks' is not yet advertised via CPUID.

Signed-off-by: Paul Durrant <pdurrant@amazon.com>
Acked-by: Wei Liu <wl@xen.org>