tpm: Check TPM XML device configuration changes after edit

tpm: Check TPM XML device configuration changes after edit

Since swtpm does not support getting started without password
once it was created with encryption enabled, we don't allow
encryption to be removed. Similarly, we do not allow encryption
to be added once swtpm has run. We also prevent chaning the type
of the TPM backend since the encrypted state is still around and
the next time one was to switch back to the emulator backend
and forgot the encryption the TPM would not work.

Signed-off-by: Stefan Berger <stefanb@linux.ibm.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>