virsh: volume: Fix lookup of volumes to provide better error messages

If a user specifies the pool explicitly, we should make sure to point
out that it's inactive instead of falling back to lookup by key/path and
failing at the end. Also if the pool isn't found there's no use in
continuing the lookup.

This changes the error in case the user-selected pool is inactive from:

 $ virsh vol-upload --pool inactivepool --vol somevolname volcontents
 error: failed to get vol 'somevolname'
 error: Storage volume not found: no storage vol with matching path
 somevolname

To a more descriptive:

 $ virsh vol-upload --pool inactivepool --vol somevolname volcontents
 error: pool 'inactivepool' is not active

And in case a user specifies an invalid pool from:

 $ virsh vol-upload --pool invalidpool --vol somevolname volcontents
 error: failed to get pool 'invalidpool'
 error: failed to get vol 'somevolname', specifying --pool might help
 error: Storage volume not found: no storage vol with matching path somevolname

To something less confusing:

 $ virsh vol-upload --pool invalidpool --vol somevolname volcontents
 error: failed to get pool 'invalidpool'
 error: Storage pool not found: no storage pool with matching name 'invalidpool'

tools/virsh-volume.c

@@ -60,8 +60,16 @@ vshCommandOptVolBy(vshControl *ctl, const vshCmd *cmd,
         vshCommandOptStringReq(ctl, cmd, pooloptname, &p) < 0)
         return NULL;
 
-    if (p)
-        pool = vshCommandOptPoolBy(ctl, cmd, pooloptname, name, flags);
+    if (p) {
+        if (!(pool = vshCommandOptPoolBy(ctl, cmd, pooloptname, name, flags)))
+            return NULL;
+
+        if (virStoragePoolIsActive(pool) != 1) {
+            vshError(ctl, _("pool '%s' is not active"), p);
+            virStoragePoolFree(pool);
+            return NULL;
+        }
+    }
 
     vshDebug(ctl, VSH_ERR_DEBUG, "%s: found option <%s>: %s\n",
              cmd->def->name, optname, n);