x86: default to reboot via ACPI

Triple-fault and keyboard reset may assert INIT instead of RESET; however
INIT is blocked when Intel VT is enabled.  This leads to a partially reset
machine when invoking emergency_restart via sysrq-b: the processor is still
working but other parts of the system are dead.

Default to rebooting via ACPI, which correctly asserts RESET and reboots the
machine.

This is safe since we will fall back to keyboard reset and triple fault if
acpi is not enabled or if the reset is not successful.

Signed-off-by: Avi Kivity <avi@qumranet.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>