KVM: X86: support paravirtualized help for TLB shootdowns

When running on a virtual machine, IPIs are expensive when the target
CPU is sleeping.  Thus, it is nice to be able to avoid them for TLB
shootdowns.  KVM can just do the flush via INVVPID on the guest's behalf
the next time the CPU is scheduled.

Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Radim Krčmář <rkrcmar@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Wanpeng Li <wanpeng.li@hotmail.com>
[Use "&" to test the bit instead of "==". - Paolo]
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Radim Krčmář <rkrcmar@redhat.com>