It's only used in qmp_chardev_add(), so use a create() argument instead.

Also switched to typedef functions for CharDriverParse/CharDriverCreate.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-Id: <20161022100951.19562-7-marcandre.lureau@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>