tests/functional: Use -no-shutdown in the hppa_seabios test

tests/functional: Use -no-shutdown in the hppa_seabios test

qemu-system-hppa shuts down automatically when the BIOS is
unable to boot from any device. So this test currently fails
occasionally when QEMU already quit, but the test still
expected it to be around (e.g. to shut it down cleanly).
Adding a "-no-shutdown" seems to make it reliable.
While we're at it, also remove the stray "self.machine" in
there that does not have any purpose.

Signed-off-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Message-id: 20250508180918.228757-1-thuth@redhat.com
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>