KVM: Move apic timer migration away from critical section

Migrating the apic timer in the critical section is not very nice, and is
absolutely horrible with the real-time port.  Move migration to the regular
vcpu execution path, triggered by a new bitflag.

Cc: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Avi Kivity <avi@qumranet.com>

diff --cc arch/x86/kvm/irq.h
Simple merge

diff --cc arch/x86/kvm/lapic.c
Simple merge

diff --cc arch/x86/kvm/x86.c
Simple merge

diff --cc include/linux/kvm_host.h
Simple merge