KVM: selftests: Unconditionally use memslot 0 when loading elf binary

Use memslot '0' for all vm_vaddr_alloc() calls when loading the test
binary.  This is the first step toward adding a helper to handle page
allocations with a default value for the target memslot.

No functional change intended.

Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20210622200529.3650424-4-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>