KVM: don't enter guest after SIPI was received by a CPU

The vcpu should process pending SIPI message before entering guest mode again.
kvm_arch_vcpu_runnable() returns true if the vcpu is in SIPI state, so
we can't call it here.

Signed-off-by: Gleb Natapov <gleb@qumranet.com>
Signed-off-by: Avi Kivity <avi@redhat.com>