virsh_shutdown: Using agent requires write acl

Need to add the "write" acl to the list since using the agent requires
it.  Like snapshot_create_as too bad there wasn't a way to add the acl
for just the agent subtest (I didn't see an easy way).
Merge pull request #207 from jferlan/virsh_shutdown_acl

virsh_shutdown: Using agent requires write acl