qerror.h: Replace QERR_NOT_SUPPORTED with QERR_UNSUPPORTED

The former is only used twice, the latter is used over 30 times, and
has a nicer error message.

Cc: Luiz Capitulino <lcapitulino@redhat.com>
Cc: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Cole Robinson <crobinso@redhat.com>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>
diff --git a/blockjob.c b/blockjob.c
index b3ce14c..cd4784f 100644
--- a/blockjob.c
+++ b/blockjob.c
@@ -88,7 +88,7 @@
     Error *local_err = NULL;
 
     if (!job->driver->set_speed) {
-        error_set(errp, QERR_NOT_SUPPORTED);
+        error_set(errp, QERR_UNSUPPORTED);
         return;
     }
     job->driver->set_speed(job, speed, &local_err);