mshv: make field names descriptive in a header struct

mshv: make field names descriptive in a header struct

When struct fields use very common names like "pages" or "type", it makes
it difficult to find uses of these fields with tools like grep, cscope,
etc when the struct is in a header file included in many places. Add
prefix mreg_ to some fields in struct mshv_mem_region to make it easier
to find them.

There is no functional change.

Signed-off-by: Mukesh R <mrathor@linux.microsoft.com>
Signed-off-by: Wei Liu <wei.liu@kernel.org>