KVM: selftests: Trust that MAXPHYADDR > memslot0 in vmx_apic_access_test

Use vm->max_gfn to compute the highest gpa in vmx_apic_access_test, and
blindly trust that the highest gfn/gpa will be well above the memory
carved out for memslot0.  The existing check is beyond paranoid; KVM
doesn't support CPUs with host.MAXPHYADDR < 32, and the selftests are all
kinds of hosed if memslot0 overlaps the local xAPIC, which resides above
"lower" (below 4gb) DRAM.

Signed-off-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>