This reverts commit 6e244c65, which
added support to qemu for the "peer" attribute in domain interface <ip>
elements.

It's being removed temporarily for the release of libvirt 1.3.4
because the feature doesn't work, and there are concerns that it may
need to be modified in an externally visible manner which could create
backward compatibility problems.

 Conflicts:
   tests/qemuxml2argvmock.c - a mock of virNetDevSetOnline() was added
   which may be assumed by other tests added since the original commit,
   so it isn't being reverted.