char-fd: remove useless chr pointer

Apparently unused since it was introduced in commit
a29753f8. Now, it can be trivially
accessed by CHARDEV() of self.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-Id: <20170720100046.4424-1-marcandre.lureau@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>