x86/jailhouse: Halt instead of failing to restart

Jailhouse provides no guest-initiated restart. So, do not even try to.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: jailhouse-dev@googlegroups.com
Link: https://lkml.kernel.org/r/ef8a0ef95c2b17c21066e5f28ea56b58bf7eaa82.1511770314.git.jan.kiszka@siemens.com