KVM: VMX: Introduce generic fastpath handler

Introduce generic fastpath handler to handle MSR fastpath, VMX-preemption
timer fastpath etc; move it after vmx_complete_interrupts() in order to
catch events delivered to the guest, and abort the fast path in later
patches.  While at it, move the kvm_exit tracepoint so that it is printed
for fastpath vmexits as well.

There is no observed performance effect for the IPI fastpath after this patch.

Tested-by: Haiwei Li <lihaiwei@tencent.com>
Cc: Haiwei Li <lihaiwei@tencent.com>
Signed-off-by: Wanpeng Li <wanpengli@tencent.com>
Suggested-by: Sean Christopherson <sean.j.christopherson@intel.com>
Message-Id: <1588055009-12677-2-git-send-email-wanpengli@tencent.com>
Reviewed-by: Vitaly Kuznetsov <vkuznets@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>