Revert "core: detach PCI devices before shutting down VM"

Many drivers, including iwlwifi doesn't handle this well, resulting in
oopses etc. Also we're disabling PCI hotplug, which may be result in
more troubles here.
This reverts commit 2658c9a6e6.

QubesOS/qubes-issues#1673

core-modules/000QubesVm.py

@@ -2057,15 +2057,6 @@ class QubesVm(object):
         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 as e:
-            print >>sys.stderr, "WARNING: {}, continuing VM shutdown " \
-                                "anyway".format(str(e))
-
         self.libvirt_domain.shutdown()
 
     def force_shutdown(self, xid = None):