build: avoid variable named 'interface', for mingw

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>