hvm/dmop: implement COPY_{TO,FROM}_GUEST_BUF_OFFSET() helpers

hvm/dmop: implement COPY_{TO,FROM}_GUEST_BUF_OFFSET() helpers

copy_{to,from}_guest_buf() are now implemented using an offset of 0.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Signed-off-by: Jennifer Herbert <Jennifer.Herbert@citrix.com>
Reviewed-by: Paul Durrant <paul.durrant@citrix.com>
This does only extend to the functionality here, specifically not to
the use of all-upper-case names for the macros:

Reviewed-by: Jan Beulich <jbeulich@suse.com>
Release-acked-by: Julien Grall <julien.grall@arm.com>