conf: Enable TPM CRB interface in the domain XML

conf: Enable TPM CRB interface in the domain XML

Enable the TPM CRB to be specified in the domain XML. This
now allows to describe the TPM device like this:

  <tpm model='tpm-crb'>
    <backend type='passthrough'>
      <device path='/dev/tpm0'/>
    </backend>
  </tpm>

Extend the XML schema to also allow tpm-crb.
Extend the documentation.
Add a test case for testing the XML parser and formatter.

Signed-off-by: Stefan Berger <stefanb@linux.vnet.ibm.com>
Reviewed-by: John Ferlan <jferlan@redhat.com>