This struct only used in whpx-apic.c, there is no need
expose it in whpx.h.

Signed-off-by: Yonggang Luo <luoyonggang@gmail.com>
Message-Id: <20210107101919.80-6-luoyonggang@gmail.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>