block-backend: convert blk_co_pwritev_part to int64_t bytes

block-backend: convert blk_co_pwritev_part to int64_t bytes

We convert blk_do_pwritev_part() and some wrappers:
blk_co_pwritev_part(), blk_co_pwritev(), blk_co_pwrite_zeroes().

All functions are converted so that the parameter type becomes wider, so
all callers should be OK with it.

Look at blk_do_pwritev_part() body:
bytes is passed to:

 - trace_blk_co_pwritev (we update it here)
 - blk_check_byte_request, throttle_group_co_io_limits_intercept,
   bdrv_co_pwritev_part - all already have int64_t argument.

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: <20211006131718.214235-4-vsementsov@virtuozzo.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
[eblake: grammar tweaks]
Signed-off-by: Eric Blake <eblake@redhat.com>