conf: allow setting peer address in <ip> element of <interface>

conf: allow setting peer address in <ip> element of <interface>

The peer attribute is used to set the property of the same name in the
interface IP info:

  <interface type='ethernet'>
    ...
    <ip family='ipv4' address='192.168.122.5'
        prefix='32' peer='192.168.122.6'/>
    ...
  </interface>

Note that this element is used to set the IP information on the
*guest* side interface, not the host side interface - that will be
supported in an upcoming patch.

(This patch now has quite a history: it was originally pushed in
commit 690969af, which was subsequently reverted in commit 1d14b13f,
then reworked and pushed (along with a lot of other related/supporting
patches) in commit 93135abf1; however *that* commit had been
accidentally pushed during dev. freeze for release 2.0.0, so it was
again reverted in commit f6acf039f0).

Signed-off-by: Vasiliy Tolstov <v.tolstov@selfip.ru>
Signed-off-by: Laine Stump <laine@laine.org>