qemu: Accept <allowReboot value='default'/>

qemu: Accept <allowReboot value='default'/>

Up until a few commits ago, libvirt produced this XML and so
we need to be able to read it back to prevent a bunch of

  error : virXMLPropEnumInternal:516 : XML error: Invalid value
  for attribute 'value' in element 'allowReboot': 'default'

messages from being logged on daemon upgrade when there are
running guests.

Fixes: 0fe2d8dd335054fae38b46bbbac58a4662e1a1d0
Signed-off-by: Andrea Bolognani <abologna@redhat.com>
Reviewed-by: Michal Privoznik <mprivozn@redhat.com>