KVM: x86: Yield to IPI target vCPU only if it is busy

KVM: x86: Yield to IPI target vCPU only if it is busy

When sending a call-function IPI-many to vCPUs, yield to the
IPI target vCPU which is marked as preempted.

but when emulating HLT, an idling vCPU will be voluntarily
scheduled out and mark as preempted from the guest kernel
perspective. yielding to idle vCPU is pointless and increase
unnecessary vmexit, maybe miss the true preempted vCPU

so yield to IPI target vCPU only if vCPU is busy and preempted

Signed-off-by: Li RongQing <lirongqing@baidu.com>
Message-Id: <1644380201-29423-1-git-send-email-lirongqing@baidu.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>