configure: Fix set-but-not-used warning in Xen 4.1 probe

The Xen 4.1 probe never uses the return value from xc_interface_open(),
so was provoking a compiler warning on newer gcc. Fix by not bothering
to put the return value anywhere.
Signed-off-by: NPeter Maydell <peter.maydell@linaro.org>
Signed-off-by: NBlue Swirl <blauwirbel@gmail.com>

configure

@@ -1431,9 +1431,8 @@ EOF
 # error HVM_MAX_VCPUS not defined
 #endif
 int main(void) {
-  xc_interface *xc;
   xs_daemon_open();
-  xc = xc_interface_open(0, 0, 0);
+  xc_interface_open(0, 0, 0);
   xc_hvm_set_mem_type(0, 0, HVMMEM_ram_ro, 0, 0);
   xc_gnttab_open(NULL, 0);
   xc_domain_add_to_physmap(0, 0, XENMAPSPACE_gmfn, 0, 0);