chardev/char.c: fix "abstract device type" error message

Current error message:

 qemu-system-x86_64: -chardev spice,id=foo: Parameter 'driver' expects an abstract device type

while in fact the meaning is in reverse, -chardev expects
a non-abstract device type.

Fixes: 777357d758d9 ("chardev: qom-ify" 2016-12-07)
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>
Reviewed-by: Zhao Liu <zhao1.liu@intel.com>

chardev/char.c

@@ -518,7 +518,7 @@ static const ChardevClass *char_get_class(const char *driver, Error **errp)
 
     if (object_class_is_abstract(oc)) {
         error_setg(errp, QERR_INVALID_PARAMETER_VALUE, "driver",
-                   "an abstract device type");
+                   "a non-abstract device type");
         return NULL;
     }