Drivers: hv: Allocate the paravisor SynIC pages when required

Drivers: hv: Allocate the paravisor SynIC pages when required

Confidential VMBus requires interacting with two SynICs -- one
provided by the host hypervisor, and one provided by the paravisor.
Each SynIC requires its own message and event pages.

Refactor and extend the existing code to add allocating and freeing
the message and event pages for the paravisor SynIC when it is
present.

Signed-off-by: Roman Kisel <romank@linux.microsoft.com>
Reviewed-by: Tianyu Lan <tiala@microsoft.com>
Reviewed-by: Michael Kelley <mhklinux@outlook.com>
Signed-off-by: Wei Liu <wei.liu@kernel.org>