x86/kvm: rename HV_X64_MSR_APIC_ASSIST_PAGE to HV_X64_MSR_VP_ASSIST_PAGE
The assist page has been used only for the paravirtual EOI so far, hence
the "APIC" in the MSR name. Renaming to match the Hyper-V TLFS where it's
called "Virtual VP Assist MSR".