block/io: support int64_t bytes in bdrv_co_do_copy_on_readv()

We are generally moving to int64_t for both offset and bytes parameters
on all io paths.

Main motivation is realization of 64-bit write_zeroes operation for
fast zeroing large disk chunks, up to the whole disk.

We chose signed type, to be consistent with off_t (which is signed) and
with possibility for signed return type (where negative value means
error).

So, prepare bdrv_co_do_copy_on_readv() now.

'bytes' type widening, so callers are safe. Look at the function
itself:

bytes, skip_bytes and progress become int64_t.

bdrv_round_to_clusters() is OK, cluster_bytes now may be large.
trace_bdrv_co_do_copy_on_readv() is OK

looping through cluster_bytes is still OK.

pnum is still capped to max_transfer, and to MAX_BOUNCE_BUFFER when we
are going to do COR operation. Therefor calculations in
qemu_iovec_from_buf() and bdrv_driver_preadv() should not change.

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Message-Id: <20201211183934.169161-13-vsementsov@virtuozzo.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Eric Blake <eblake@redhat.com>