utils_test.libvirt: Add reset_domain function

The reset_domain function is for manipulate domain state, guest agent
setup, domain disk replacement, etc. As each case with differ needs
for pre-test domain configuration, several cases manipulate the
function differently in specific case, so this is not a got all in
solution for now.

The domain disk replacement is new here, which take a lot of usage
with functions exist in current module, e.g. setup iscsi, nfs, gluster
for storage backend.

Disk replacement is for cover different disk types as supported in
domain xml, including file type(local, nfs), network type(gluster,
iscsi), block type(use connected iscsi block disk). This is different
than setting up pool first and use the vol in the pool, which covered
in pool related testing.

Signed-off-by: Wayne Sun <gsun@redhat.com>