blockcommit: Add snpashot during active-commit negative case

When blockcommit without --pivot, block job will still be
active-commit, doing snapshot at this phase will fail.

Signed-off-by: Wayne Sun <gsun@redhat.com>
blockcommit: Add block disk type test cases

For block disk in vm, use connected iscsi block disk. Before
snapshot and block commit, setup iscsi env and copy vm os disk
to iscsi backing store file which will enable vm booted with
full os, then start vm and do testing.

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