block: use 'unsigned' for in_flight field on driver state

This patch makes in_flight field 'unsigned' for BDRVNBDState and
MirrorBlockJob. This matches the definition of this field on BDS
and is generically correct - we should never get negative value here.

Signed-off-by: Denis V. Lunev <den@openvz.org>
CC: John Snow <jsnow@redhat.com>
CC: Vladimir Sementsov-Ogievskiy <vsementsov@yandex-team.ru>
CC: Kevin Wolf <kwolf@redhat.com>
CC: Hanna Reitz <hreitz@redhat.com>
CC: Eric Blake <eblake@redhat.com>
Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@yandex-team.ru>
Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@yandex-team.ru>