KVM: selftests: Unconditionally use memslot 0 for x86's GDT/TSS setup

Refactor x86's GDT/TSS allocations to for memslot '0' at its
vm_addr_alloc() call sites instead of passing in '0' from on high.  This
is a step toward using a common helper for allocating pages.

No functional change intended.

Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20210622200529.3650424-5-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>