build: avoid variable named 'interface', for mingw
    
    Commit 2f36e6944 (re-)introduced a use of an identifier 'interface',
    which causes this build failure on mingw:
    
    ../../tools/virsh-domain-monitor.c: In function 'cmdDomIfAddr':
    ../../tools/virsh-domain-monitor.c:2233:17: error: expected identifier or '(' before 'struct'
         const char *interface = NULL;
                          ^
    
    See also commit 6512c8b.  Sadly, I'm not quite sure how to write a
    syntax check that can poison the use of this identifier.
    
    * tools/virsh-domain-monitor.c (cmdDomIfAddr): Use ifacestr instead.
    
    Signed-off-by: Eric Blake <eblake@redhat.com>