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>