KVM: VMX: Use vcpu_to_pi_desc() uniformly in posted_intr.c

The helper function, vcpu_to_pi_desc(), is defined to get the posted
interrupt descriptor from vcpu.  There is one place that doesn't use
it, and instead references vmx_vcpu->pi_desc directly.  Remove the
inconsistency.

Signed-off-by: Yuan Yao <yuan.yao@intel.com>
Signed-off-by: Isaku Yamahata <isaku.yamahata@intel.com>
Message-Id: <ee7be7832bc424546fd4f05015a844a0205b5ba2.1646422845.git.isaku.yamahata@intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>