viridian: add ExProcessorMasks variants of the flush hypercalls

viridian: add ExProcessorMasks variants of the flush hypercalls

The Microsoft Hypervisor TLFS specifies variants of the already implemented
HVCALL_FLUSH_VIRTUAL_ADDRESS_SPACE/LIST hypercalls that take a 'Virtual
Processor Set' as an argument rather than a simple 64-bit mask.

This patch adds a new hvcall_flush_ex() function to implement these
(HVCALL_FLUSH_VIRTUAL_ADDRESS_SPACE/LIST_EX) hypercalls. This makes use of
new helper functions, hv_vpset_nr_banks() and hv_vpset_to_vpmask(), to
determine the size of the Virtual Processor Set (so it can be copied from
guest memory) and parse it into hypercall_vpmask (respectively).

NOTE: A guest should not yet issue these hypercalls as 'ExProcessorMasks'
      support needs to be advertised via CPUID. This will be done in a
      subsequent patch.

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