Instead of passing a single buffer pointer to do_perform_cow_write(),
pass a QEMUIOVector. This will allow us to merge the write requests
for the COW regions and the actual data into a single one.

Although do_perform_cow_read() does not strictly need to change its
API, we're doing it here as well for consistency.

Signed-off-by: Alberto Garcia <berto@igalia.com>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>