The check for a network being active during interface attach was being
done individually in several places (by both the lxc driver and the
qemu driver), but those places were too specific, leading to it *not*
being checked when allocating a connection/device from a macvtap or
hostdev network.

This patch puts a single check in networkAllocateActualDevice(), which
is always called before the any network interface is attached to any
type of domain. It also removes all the other now-redundant checks
from the lxc and qemu drivers.

NB: the following patches are prerequisites for this patch, in the
case that it is backported to any branch:

  440beeb7 network: fix virNetworkObjAssignDef and persistence
  8aaa5b68 network: create statedir during driver initialization
  b9e95491 network: change location of network state xml files
  411c5486 network: set macvtap/hostdev networks active if their state
          file exists

This fixes:

  https://bugzilla.redhat.com/show_bug.cgi?id=880483