block: do not treat disks of not running VMs as used

There are legitimate use cases when one want to attach disk of one VM to
some other. Do not try to detach the disk from powered down VM in such
case.

core/qubesutils.py

@@ -337,6 +337,8 @@ def block_check_attached(qvmc, device):
         if vm.qid == 0:
             # Connecting devices to dom0 not supported
             continue
+        if not vm.is_running():
+            continue
         try:
             libvirt_domain = vm.libvirt_domain
             if libvirt_domain: