Handle init/sipi in a main cpu exec loop. (v2)

This should fix compilation problem in case of CONFIG_USER_ONLY.

Currently INIT/SIPI is handled in the context of CPU that sends IPI.
This patch changes this to handle them like all other events in a main
cpu exec loop. When KVM will gain thread per vcpu capability it will
be much more clear to handle those event by cpu thread itself and not
modify one cpu's state from the context of the other.

Signed-off-by: Gleb Natapov <gleb@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
