mirror: double performance of the bulk stage if the disc is full

Mirror can do up to 16 in-flight requests, but actually on full copy
(the whole source disk is non-zero) in-flight is always 1. This happens
as the request is not limited in size: the data occupies maximum available
capacity of s->buf.

The patch limits the size of the request to some artificial constant
(1 Mb here), which is not that big or small. This effectively enables
back parallelism in mirror code as it was designed.

The result is important: the time to migrate 10 Gb disk is reduced from
~350 sec to 170 sec.

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Signed-off-by: Denis V. Lunev <den@openvz.org>
Reviewed-by: Max Reitz <mreitz@redhat.com>
Reviewed-by: Jeff Cody <jcody@redhat.com>
Message-id: 1468516741-82174-1-git-send-email-vsementsov@virtuozzo.com
CC: Stefan Hajnoczi <stefanha@redhat.com>
CC: Fam Zheng <famz@redhat.com>
CC: Kevin Wolf <kwolf@redhat.com>
CC: Max Reitz <mreitz@redhat.com>
CC: Jeff Cody <jcody@redhat.com>
CC: Eric Blake <eblake@redhat.com>
Signed-off-by: Jeff Cody <jcody@redhat.com>