viridian: add implementation of the HvSendSyntheticClusterIpi hypercall

viridian: add implementation of the HvSendSyntheticClusterIpi hypercall

This patch adds an implementation of the hypercall as documented in the
specification [1], section 10.5.2. This enlightenment, as with others, is
advertised by CPUID leaf 0x40000004 and is under control of a new
'hcall_ipi' option in libxl.

If used, this enlightenment should mean the guest only takes a single VMEXIT
to issue IPIs to multiple vCPUs rather than the multiple VMEXITs that would
result from using the emulated local APIC.

[1] https://github.com/MicrosoftDocs/Virtualization-Documentation/raw/live/tlfs/Hypervisor%20Top%20Level%20Functional%20Specification%20v5.0C.pdf

Signed-off-by: Paul Durrant <paul.durrant@citrix.com>
Acked-by: Wei Liu <wei.liu2@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>