utils/QubesWatch: register libvirt event loop only when really launched

Registering event implementation in libvirt and then not calling it is
harmful, because libvirt expects it working. Known drawbacks:
- keep-alives are advertised as supported but not really sent (cause
  dropping connections)
- connections are not closed (sockets remains open, effectively leaking
  file descriptors)

So call libvirt.virEventRegisterDefaultImpl only when it will be really
used (libvirt.virEventRunDefaultImpl called), which means calling it in
QubesWatch. Registering events implementation have effect only on new
libvirt connections, so start a new one for QubesWatch.

Fixes QubesOS/qubes-issues#1380

core/qubes.py

@@ -159,7 +159,6 @@ class QubesVMMConnection(object):
 
         if 'xen.lowlevel.xs' in sys.modules:
             self._xs = xen.lowlevel.xs.xs()
-        libvirt.virEventRegisterDefaultImpl()
         self._libvirt_conn = libvirt.open(defaults['libvirt_uri'])
         if self._libvirt_conn == None:
             raise QubesException("Failed connect to libvirt driver")

core/qubesutils.py

@@ -29,7 +29,7 @@ from lxml import etree
 from lxml.etree import ElementTree, SubElement, Element
 
 from qubes.qubes import QubesException
-from qubes.qubes import vmm
+from qubes.qubes import vmm,defaults
 from qubes.qubes import system_path,vm_files
 import sys
 import os
@@ -737,11 +737,16 @@ class QubesWatch(object):
         self.block_callback = None
         self.meminfo_callback = None
         self.domain_callback = None
-        vmm.libvirt_conn.domainEventRegisterAny(
+        libvirt.virEventRegisterDefaultImpl()
+        # open new libvirt connection because above
+        # virEventRegisterDefaultImpl is in practice effective only for new
+        # connections
+        self.libvirt_conn = libvirt.open(defaults['libvirt_uri'])
+        self.libvirt_conn.domainEventRegisterAny(
             None,
             libvirt.VIR_DOMAIN_EVENT_ID_LIFECYCLE,
             self._domain_list_changed, None)
-        vmm.libvirt_conn.domainEventRegisterAny(
+        self.libvirt_conn.domainEventRegisterAny(
             None,
             libvirt.VIR_DOMAIN_EVENT_ID_DEVICE_REMOVED,
             self._device_removed, None)