Add a virGetLastErrorMessage() function

Apps using libvirt will often have code like

   if (virXXXX() < 0) {
      virErrorPtr err = virGetLastError();
      fprintf(stderr, "Something failed: %s\n",
              err && err->message ? err->message :
              "unknown error");
      return -1;
   }

Checking for a NULL error object or message leads to very
verbose code. A virGetLastErrorMessage() helper from libvirt
can simplify this to

   if (virXXXX() < 0) {
      fprintf(stderr, "Something failed: %s\n",
              virGetLastErrorMessage());
      return -1;
   }

Signed-off-by: Daniel P. Berrange <berrange@redhat.com>
(cherry picked from commit 69c6a58a1d)

include/libvirt/virterror.h

@@ -313,6 +313,8 @@ void			virResetLastError	(void);
 void			virResetError		(virErrorPtr err);
 void			virFreeError		(virErrorPtr err);
 
+const char *            virGetLastErrorMessage  (void);
+
 virErrorPtr		virConnGetLastError	(virConnectPtr conn);
 void			virConnResetLastError	(virConnectPtr conn);
 int			virCopyLastError	(virErrorPtr to);

src/libvirt_public.syms

@@ -616,4 +616,9 @@ LIBVIRT_1.0.5 {
         virNodeDeviceDetachFlags;
 } LIBVIRT_1.0.3;
 
+LIBVIRT_1.0.6 {
+    global:
+        virGetLastErrorMessage;
+} LIBVIRT_1.0.5;
+
 # .... define new API here using predicted next version number ....

src/util/virerror.c

@@ -233,6 +233,27 @@ virGetLastError(void)
     return err;
 }
 
+
+/**
+ * virGetLastErrorMessage:
+ *
+ * Get the most recent error message
+ *
+ * Returns the most recent error message string in this
+ * thread, or a generic message if none is set
+ */
+const char *
+virGetLastErrorMessage(void)
+{
+    virErrorPtr err = virLastErrorObject();
+    if (!err || err->code == VIR_ERR_OK)
+        return _("no error");
+    if (err->message == NULL)
+        return _("unknown error");
+    return err->message;
+}
+
+
 /**
  * virSetError:
  * @newerr: previously saved error object