x86/hyperv: Set Virtual Trust Level in VMBus init message

SEV-SNP guests on Hyper-V can run at multiple Virtual Trust
Levels (VTL).  During boot, get the VTL at which we're running
using the GET_VP_REGISTERs hypercall, and save the value
for future use.  Then during VMBus initialization, set the VTL
with the saved value as required in the VMBus init message.

Reviewed-by: Dexuan Cui <decui@microsoft.com>
Reviewed-by: Michael Kelley <mikelley@microsoft.com>
Signed-off-by: Tianyu Lan <tiala@microsoft.com>
Signed-off-by: Wei Liu <wei.liu@kernel.org>
Link: https://lore.kernel.org/r/20230818102919.1318039-3-ltykernel@gmail.com