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

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>
(cherry picked from commit 4ad87cd4b2254197b7ac12e3da824854e6a90f8f)
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>