KVM: selftests: Unconditionally use memslot '0' for page table allocations

Drop the memslot param from virt_pg_map() and virt_map() and shove the
hardcoded '0' down to the vm_phy_page_alloc() calls.

No functional change intended.

Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20210622200529.3650424-13-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>