hv: hyperv.h: Introduce some hvpfn helper functions

hv: hyperv.h: Introduce some hvpfn helper functions

When a guest communicate with the hypervisor, it must use HV_HYP_PAGE to
calculate PFN, so introduce a few hvpfn helper functions as the
counterpart of the page helper functions. This is the preparation for
supporting guest whose PAGE_SIZE is not 4k.

Signed-off-by: Boqun Feng <boqun.feng@gmail.com>
Reviewed-by: Michael Kelley <mikelley@microsoft.com>
Link: https://lore.kernel.org/r/20200916034817.30282-7-boqun.feng@gmail.com
Signed-off-by: Wei Liu <wei.liu@kernel.org>