block/io: support int64_t bytes in bdrv_aligned_pwritev()

block/io: support int64_t bytes in bdrv_aligned_pwritev()

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_aligned_pwritev() now and convert the dependencies:
bdrv_co_write_req_prepare() and bdrv_co_write_req_finish() to signed
type bytes.

Conversion of bdrv_co_write_req_prepare() and
bdrv_co_write_req_finish() is definitely safe, as all requests in
block/io must not overflow BDRV_MAX_LENGTH. Still add assertions.

For bdrv_aligned_pwritev() 'bytes' type is widened, so callers are
safe. Let's check usage of the parameter inside the function.

Passing to bdrv_co_write_req_prepare() and bdrv_co_write_req_finish()
is OK.

Passing to qemu_iovec_* is OK after new assertion. All other callees
are already updated to int64_t.

Checking alignment is not changed, offset + bytes and qiov_offset +
bytes calculations are safe (thanks to new assertions).

max_transfer is kept to be int for now. It has a default of INT_MAX
here, and some drivers may rely on it. It's to be refactored later.

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Message-Id: <20201211183934.169161-12-vsementsov@virtuozzo.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Eric Blake <eblake@redhat.com>