virsh: report errors for empty strings

Several callers were using vshCommandOptString without setting an error.
Use vshCommandOptStringReq which sets the error.

https://bugzilla.redhat.com/show_bug.cgi?id=1281707
(cherry picked from commit 45138725)

tools/virsh-domain-monitor.c

@@ -2237,9 +2237,9 @@ cmdDomIfAddr(vshControl *ctl, const vshCmd *cmd)
     if (!(dom = vshCommandOptDomain(ctl, cmd, NULL)))
         return false;
 
-    if (vshCommandOptString(ctl, cmd, "interface", &ifacestr) < 0)
+    if (vshCommandOptStringReq(ctl, cmd, "interface", &ifacestr) < 0)
         goto cleanup;
-    if (vshCommandOptString(ctl, cmd, "source", &sourcestr) < 0)
+    if (vshCommandOptStringReq(ctl, cmd, "source", &sourcestr) < 0)
         goto cleanup;
 
     if (sourcestr) {

tools/virsh-domain.c

@@ -2275,11 +2275,11 @@ cmdBlockCopy(vshControl *ctl, const vshCmd *cmd)
 
     if (vshCommandOptStringReq(ctl, cmd, "path", &path) < 0)
         return false;
-    if (vshCommandOptString(ctl, cmd, "dest", &dest) < 0)
+    if (vshCommandOptStringReq(ctl, cmd, "dest", &dest) < 0)
         return false;
-    if (vshCommandOptString(ctl, cmd, "xml", &xml) < 0)
+    if (vshCommandOptStringReq(ctl, cmd, "xml", &xml) < 0)
         return false;
-    if (vshCommandOptString(ctl, cmd, "format", &format) < 0)
+    if (vshCommandOptStringReq(ctl, cmd, "format", &format) < 0)
         return false;
     /* XXX: Parse bandwidth as scaled input, rather than forcing
      * MiB/s, and either reject negative input or treat it as 0 rather
@@ -9146,7 +9146,7 @@ cmdQemuMonitorEvent(vshControl *ctl, const vshCmd *cmd)
     data.count = 0;
     if (vshCommandOptTimeoutToMs(ctl, cmd, &timeout) < 0)
         return false;
-    if (vshCommandOptString(ctl, cmd, "event", &event) < 0)
+    if (vshCommandOptStringReq(ctl, cmd, "event", &event) < 0)
         return false;
 
     if (vshCommandOptBool(cmd, "domain"))
@@ -12368,7 +12368,7 @@ cmdEvent(vshControl *ctl, const vshCmd *cmd)
         return true;
     }
 
-    if (vshCommandOptString(ctl, cmd, "event", &eventName) < 0)
+    if (vshCommandOptStringReq(ctl, cmd, "event", &eventName) < 0)
         return false;
     if (eventName) {
         for (event = 0; event < VIR_DOMAIN_EVENT_ID_LAST; event++)

tools/virsh-network.c

@@ -1256,7 +1256,7 @@ cmdNetworkEvent(vshControl *ctl, const vshCmd *cmd)
         return true;
     }
 
-    if (vshCommandOptString(ctl, cmd, "event", &eventName) < 0)
+    if (vshCommandOptStringReq(ctl, cmd, "event", &eventName) < 0)
         return false;
     if (!eventName) {
         vshError(ctl, "%s", _("either --list or event type is required"));
@@ -1366,7 +1366,7 @@ cmdNetworkDHCPLeases(vshControl *ctl, const vshCmd *cmd)
     unsigned int flags = 0;
     virNetworkPtr network = NULL;
 
-    if (vshCommandOptString(ctl, cmd, "mac", &mac) < 0)
+    if (vshCommandOptStringReq(ctl, cmd, "mac", &mac) < 0)
         return false;
 
     if (!(network = vshCommandOptNetwork(ctl, cmd, &name)))