mirror: make sectors_in_flight int64_t

mirror: make sectors_in_flight int64_t

We keep here the sum of int fields. Thus this could easily overflow,
especially when we will start sending big requests in next patches.

Signed-off-by: Denis V. Lunev <[email protected]>
Reviewed-by: Vladimir Sementsov-Ogievskiy <[email protected]>
Reviewed-by: Eric Blake <[email protected]>
Reviewed-by: John Snow <[email protected]>
Reviewed-by: Fam Zheng <[email protected]>
Message-id: 1468503209[email protected]
CC: Stefan Hajnoczi <[email protected]>
CC: Kevin Wolf <[email protected]>
CC: Max Reitz <[email protected]>
CC: Jeff Cody <[email protected]>
Signed-off-by: Jeff Cody <[email protected]>