iscsi boot:using self.ROOT_SIZE instead of hardcoding during create_hostvg()

Change-Id: I2eae03c99742d60102c86482f5d67ab144a81da9
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=1008377
Signed-off-by: hadong <hadong0720@gmail.com>