util: use string libvirt to prefix error message instead of libvir

BZ:https://bugzilla.redhat.com/show_bug.cgi?id=912021
Without error handler set, virDefaultErrorFunc will be called, the
error message is prefixed with "libvir:". It become a little better
by using prefix "libvirt:" when working with upper application.

For example:
1, stop libvirtd daemon
2, run virt-top.
libvir: XML-RPC error : Failed to connect \
        socket to '/var/run/libvirt/libvirt-sock-ro': \
        No such file or directory
libvirt: VIR_ERR_SYSTEM_ERROR: VIR_FROM_RPC: \
        Failed to connect socket to '/var/run/libvirt/libvirt-sock-ro': \
        No such file or directory

src/util/virerror.c

@@ -525,13 +525,13 @@ virDefaultErrorFunc(virErrorPtr err)
     len = strlen(err->message);
     if ((err->domain == VIR_FROM_XML) && (err->code == VIR_ERR_XML_DETAIL) &&
         (err->int1 != 0))
-        fprintf(stderr, "libvir: %s %s %s%s: line %d: %s",
+        fprintf(stderr, "libvirt: %s %s %s%s: line %d: %s",
                 dom, lvl, domain, network, err->int1, err->message);
     else if ((len == 0) || (err->message[len - 1] != '\n'))
-        fprintf(stderr, "libvir: %s %s %s%s: %s\n",
+        fprintf(stderr, "libvirt: %s %s %s%s: %s\n",
                 dom, lvl, domain, network, err->message);
     else
-        fprintf(stderr, "libvir: %s %s %s%s: %s",
+        fprintf(stderr, "libvirt: %s %s %s%s: %s",
                 dom, lvl, domain, network, err->message);
 }