block-backend: convert blk_co_copy_range to int64_t bytes

block-backend: convert blk_co_copy_range to int64_t bytes

Function is updated so that parameter type becomes wider, so all
callers should be OK with it.

Look at blk_co_copy_range() itself: bytes is passed only to
blk_check_byte_request() and bdrv_co_copy_range(), which already have
int64_t bytes parameter, so we are OK.

Note that requests exceeding INT_MAX are still restricted by
blk_check_byte_request().

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Message-Id: <20211006131718.214235-10-vsementsov@virtuozzo.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
[eblake: grammar tweaks]
Signed-off-by: Eric Blake <eblake@redhat.com>