KVM: selftests: Convert lib's mem regions to KVM_SET_USER_MEMORY_REGION2

Use KVM_SET_USER_MEMORY_REGION2 throughout KVM's selftests library so that
support for guest private memory can be added without needing an entirely
separate set of helpers.

Note, this obviously makes selftests backwards-incompatible with older KVM
versions from this point forward.

Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20231027182217.3615211-26-seanjc@google.com>
Reviewed-by: Fuad Tabba <tabba@google.com>
Tested-by: Fuad Tabba <tabba@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>