chardev: Drop useless ChardevDummy type

Commit d0d7708b made ChardevDummy be an empty wrapper type around
ChardevCommon.  But there is no technical reason for this indirection,
so simplify the code by directly using the base type.

Also change the fallback assignment to assign u.null rather than
u.data, since a future patch will remove the data member of the C
struct generated for QAPI unions.

Signed-off-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Daniel P. Berrange <berrange@redhat.com>
Message-Id: <1457106160-23614-1-git-send-email-eblake@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>