block/block-copy: add memory limit

Currently total allocation for parallel requests to block-copy instance
is unlimited. Let's limit it to 128 MiB.

For now block-copy is used only in backup, so actually we limit total
allocation for backup job.

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>
Message-id: 20191022111805.3432-6-vsementsov@virtuozzo.com
Signed-off-by: Max Reitz <mreitz@redhat.com>