core: detach PCI devices before shutting down VM

When VM is shutting down it doesn't disconnect PCI frontend (?), so when
VM is destroyed it ends up in timeouts in PCI backend shutdown (which
can't communicate with frontend at that stage). Prevent this by
detaching PCI devices while VM is still running.

Fixes QubesOS/qubes-issues#1494
Fixes QubesOS/qubes-issues#1425

core-modules/000QubesVm.py

@@ -1948,6 +1948,14 @@ def shutdown(self, force=False, xid = None):
         if not self.is_running():
             raise QubesException ("VM already stopped!")
 
+        # try to gracefully detach PCI devices before shutdown, to mitigate
+        # timeouts on forcible detach at domain destroy; if that fails, too bad
+        try:
+            for pcidev in self.pcidevs:
+                self.libvirt_domain.detachDevice(self._format_pci_dev(pcidev))
+        except libvirt.libvirtError:
+            pass
+
         self.libvirt_domain.shutdown()
 
     def force_shutdown(self, xid = None):