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.
FixesQubesOS/qubes-issues#1380