KVM: X86: Introduce kvm_vcpu_exit_request() helper

Introduce kvm_vcpu_exit_request() helper, we need to check some conditions
before enter guest again immediately, we skip invoking the exit handler and
go through full run loop if complete fastpath but there is stuff preventing
we enter guest again immediately.

Tested-by: Haiwei Li <lihaiwei@tencent.com>
Cc: Haiwei Li <lihaiwei@tencent.com>
Signed-off-by: Wanpeng Li <wanpengli@tencent.com>
Message-Id: <1588055009-12677-5-git-send-email-wanpengli@tencent.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>