xen/setup: Route halt operations to safe_halt pvop.

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: Daniel De Graaf <dgdegra@tycho.nsa.gov>
Tested-by: Daniel De Graaf <dgdegra@tycho.nsa.gov>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>