vdsm: fix config comment for migration_timeout

The migration_timeout variable means not overall migration timeout,
but the timeout since migration is stalled. Each time when migration
does some progress, this timeout counter is reset. Thus, overall
migration time can be longer than migration_timeout, and this is
by design.

Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=970645
Signed-off-by: Peter V. Saveliev <peet@redhat.com>
Change-Id: Ic8f026d881e58016e43e654d1bbec2ddeeb1c679
Reviewed-on: http://gerrit.ovirt.org/16382
Reviewed-by: Dan Kenigsberg <danken@redhat.com>
Tested-by: Dan Kenigsberg <danken@redhat.com>