This patch adds support for an external swtpm TPM emulator. The XML for
this type of TPM looks as follows:

 <tpm model='tpm-tis'>
   <backend type='emulator'/>
 </tpm>

The XML will currently only define a TPM 1.2.

Extend the documentation.

Add a test case testing the XML parser and formatter.
Signed-off-by: NStefan Berger <stefanb@linux.vnet.ibm.com>
Reviewed-by: NJohn Ferlan <jferlan@redhat.com>
Reviewed-by: NJán Tomko <jtomko@redhat.com>