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>
blockcommit: Add file disk type with image on mounted nfs dir

Replace domain image with image file on mounted nfs dir, then
do snapshot and block commit testing.

Signed-off-by: Wayne Sun <gsun@redhat.com>
blockcommit: Use functions from utils_test sub-module

Split the reset_domain function and moved it to utils_test.libvirt
module. Then use new set_guest_agent and set_vm_disk for pre-test
configuration.

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