lib: Remove the BUSY state.

Originally this state was intended so that in some way you could find
out if the appliance was running a command.  However there was never a
thread-safe way to access the state of the handle, so in effect you
could never do anything useful safely with this information.

This commit completely removes the BUSY state.

The only visible change is to the guestfs_is_busy API.  Previously you
could never call this safely from another thread.  If you called it
from the same thread it would always return false (since the current
thread can't be running a libguestfs command at that point by
definition).  Now it always returns false.

perl/t/006-pod-coverage.t

@@ -22,7 +22,10 @@ use warnings;
 eval "use Test::Pod::Coverage 1.00";
 plan skip_all => "Test::Pod::Coverage 1.00 required for testing POD" if $@;
 all_pod_coverage_ok ({
-    also_private => [ qr/^test0.*/,
-                      qr/^debug.*/,
-                      qr/^internal.*/ ]
+    also_private => [
+        qr/^debug.*/,
+        qr/^is_busy$/,
+        qr/^internal.*/,
+        qr/^test0.*/,
+        ]
 });