tests: add XML schema tests for the TPM Proxy device

tests: add XML schema tests for the TPM Proxy device

This tests aims to exercise how a TPM Proxy device can be
added in the domain, either alone or with a regular TPM
device. It also ensures that we do not allow bogus scenarios
to slip by.

Tested-by: Satheesh Rajendran <sathnaga@linux.vnet.ibm.com>
Reviewed-by: Stefan Berger <stefanb@linux.ibm.com>
Signed-off-by: Daniel Henrique Barboza <danielhb413@gmail.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>
Signed-off-by: Ján Tomko <jtomko@redhat.com>