qga: return a more explicit error on why a command is disabled

qmp_disable_command() now takes an optional error string to return a
more explicit error message.

Fixes: https://bugzilla.redhat.com/show_bug.cgi?id=1928806



Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
*fix up 80+ char line
Signed-off-by: Michael Roth <michael.roth@amd.com>