modpost: refactor do_vmbus_entry()

Optimize the size of guid_name[], as it only requires 1 additional byte
for '\0' instead of 2.

Simplify the loop by incrementing the iterator by 1 instead of 2.

Remove the unnecessary TO_NATIVE() call, as the guid is represented as
a byte stream.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Tested-by: John Paul Adrian Glaubitz <glaubitz@physik.fu-berlin.de>