guestcopy: evaluate {,__}copy{,_field}_to_guest*() ptr argument just once

guestcopy: evaluate {,__}copy{,_field}_to_guest*() ptr argument just once

There's nothing wrong with having e.g.

    copy_to_guest(uarg, ptr++, 1);

yet until now this would increment "ptr" twice.

Also drop a pair of unneeded parentheses from every instance at this
occasion.

Fixes: b7954cc59831 ("Enhance guest memory accessor macros so that source operands can be")
Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Julien Grall <jgrall@amazon.com>