xenconfig: Properly check retval of virDomainGraphicsListenSetAddress

The function, like others in our code, returns zero on success
and a negative value on error. However, there are two places in
xenconfig source code where we check for non-zero value. While
the function can't currently return a positive value, those
checks look okay, but does not really follow our style.
Signed-off-by: NMichal Privoznik <mprivozn@redhat.com>

src/xenconfig/xen_sxpr.c

@@ -868,7 +868,7 @@ xenParseSxprGraphicsOld(virDomainDefPtr def,
         graphics->data.vnc.port = port;
 
         if (listenAddr &&
-            virDomainGraphicsListenSetAddress(graphics, 0, listenAddr, -1, true))
+            virDomainGraphicsListenSetAddress(graphics, 0, listenAddr, -1, true) < 0)
             goto error;
 
         if (VIR_STRDUP(graphics->data.vnc.auth.passwd, vncPasswd) < 0)
@@ -987,7 +987,7 @@ xenParseSxprGraphicsNew(virDomainDefPtr def,
                 graphics->data.vnc.port = port;
 
                 if (listenAddr &&
-                    virDomainGraphicsListenSetAddress(graphics, 0, listenAddr, -1, true))
+                    virDomainGraphicsListenSetAddress(graphics, 0, listenAddr, -1, true) < 0)
                     goto error;
 
                 if (VIR_STRDUP(graphics->data.vnc.auth.passwd, vncPasswd) < 0)