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 is an updated *re*-commit of commit 690969af, which was
subsequently reverted in commit 1d14b13f).

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