blockcommit: Add test cases for iscsi network disk

For testing blockcommit with iscsi network disk vm, update domain
xml with iscsi network disk with reset_domain function.

The test aim to test iscsi disk with full os, so after iscsi lun
created, convert vm disk to the lun backing store raw file on
server use qemu-img command.

Signed-off-by: Wayne Sun <gsun@redhat.com>