mshv: Add ioctl for self targeted passthrough hvcalls

mshv: Add ioctl for self targeted passthrough hvcalls

Allow MSHV_ROOT_HVCALL IOCTL on the /dev/mshv fd. This IOCTL would
execute a passthrough hypercall targeting the root/parent partition
i.e. HV_PARTITION_ID_SELF.

This will be useful for the VMM to query things like supported
synthetic processor features, supported VMM capabiliites etc.

Since hypercalls targeting the host partition could potentially perform
privileged operations, allow only a limited set of hypercalls. To begin
with, allow only:

	HVCALL_GET_PARTITION_PROPERTY
	HVCALL_GET_PARTITION_PROPERTY_EX

Signed-off-by: Anirudh Rayabharam (Microsoft) <anirudh@anirudhrb.com>
Reviewed-by: Nuno Das Neves <nunodasneves@linux.microsoft.com>
Signed-off-by: Wei Liu <wei.liu@kernel.org>