KVM: selftests: Drop vm_create_default* helpers

Drop all vm_create_default*() helpers, the "default" naming turned out to
terrible as wasn't extensible (hard to have multiple defaults), was a lie
(half the settings were default, half weren't), and failed to capture
relationships between helpers, e.g. compared with the kernel's standard
underscores pattern.

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