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'>
        ...
        
            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>