xen/setup: Route halt operations to safe_halt pvop.

With this patch, the cpuidle driver does not load and
does not issue the mwait operations. Instead the hypervisor
is doing them (b/c we call the safe_halt pvops call).

This fixes quite a lot of bootup issues wherein the user had
to force interrupts for the continuation of the bootup.

Details are discussed in:

http://lists.xensource.com/archives/html/xen-devel/2011-01/msg00535.html

[v2: Wrote the commit description]
Reported-by: NDaniel De Graaf <dgdegra@tycho.nsa.gov>
Tested-by: NDaniel De Graaf <dgdegra@tycho.nsa.gov>
Signed-off-by: NKonrad Rzeszutek Wilk <konrad.wilk@oracle.com>

arch/x86/xen/setup.c

@@ -355,6 +355,7 @@ void __init xen_arch_setup(void)
 	boot_cpu_data.hlt_works_ok = 1;
 #endif
 	pm_idle = default_idle;
+	boot_option_idle_override = IDLE_HALT;
 
 	fiddle_vdso();
 }