qemu: Support reboot command in guest

qemu: Support reboot command in guest

We can reboot a TDX guest with 'virsh reboot' or 'virsh shutdown' if action
for onPoweroff is 'restart'. But running reboot command in guest shell will
always lead to shutdown.

This behavior is not consistent with normal guest, fix it by checking
shutdown reason and action configuration to trigger FakeReboot.

Signed-off-by: Zhenzhong Duan <zhenzhong.duan@intel.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>