virerror: s/VIR_ERR_STORAGE_VOL_EXISTS/VIR_ERR_STORAGE_VOL_EXISTS/

We currently have other error codes in singular form, e.g.
VIR_ERR_NETWORK_EXIST. Cleanup the previous patch to match the form.
Signed-off-by: NMichal Privoznik <mprivozn@redhat.com>

include/libvirt/virterror.h

@@ -296,7 +296,7 @@ typedef enum {
     VIR_ERR_ACCESS_DENIED = 88,         /* operation on the object/resource
                                            was denied */
     VIR_ERR_DBUS_SERVICE = 89,          /* error from a dbus service */
-    VIR_ERR_STORAGE_VOL_EXISTS = 90,         /* the storage vol already exists */
+    VIR_ERR_STORAGE_VOL_EXIST = 90,     /* the storage vol already exists */
 } virErrorNumber;
 
 /**

src/storage/storage_driver.c

@@ -1538,7 +1538,7 @@ storageVolCreateXML(virStoragePoolPtr obj,
         goto cleanup;
 
     if (virStorageVolDefFindByName(pool, voldef->name)) {
-        virReportError(VIR_ERR_STORAGE_VOL_EXISTS,
+        virReportError(VIR_ERR_STORAGE_VOL_EXIST,
                        _("'%s'"), voldef->name);
         goto cleanup;
     }

src/util/virerror.c

@@ -1004,7 +1004,7 @@ virErrorMsg(virErrorNumber error, const char *info)
             else
                 errmsg = _("Storage volume not found: %s");
             break;
-        case VIR_ERR_STORAGE_VOL_EXISTS:
+        case VIR_ERR_STORAGE_VOL_EXIST:
             if (info == NULL)
                 errmsg = _("this storage volume exists already");
             else