conf: Add support for keeping TPM emulator state

Currently, swtpm TPM state file is removed when a transient domain is
powered off or undefined. When we store TPM state on a shared storage
such as NFS and use transient domain, TPM states should be kept as it is.

Add per-TPM emulator option `persistent_sate` for keeping TPM state.
This option only works for the emulator type backend and looks as follows:

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

Signed-off-by: Eiichi Tsukata <eiichi.tsukata@nutanix.com>
Reviewed-by: Stefan Berger <stefanb@linux.ibm.com>
Reviewed-by: Daniel Henrique Barboza <danielhb413@gmail.com>
Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Michal Privoznik <mprivozn@redhat.com>

docs/formatdomain.rst

@@ -6986,6 +6986,13 @@ Example: usage of the TPM Emulator
    -  '1.2' : creates a TPM 1.2
    -  '2.0' : creates a TPM 2.0
 
+``persistent_state``
+   The ``persistent_state`` attribute indicates whether 'swtpm' TPM state is
+   kept or not when a transient domain is powered off or undefined. This
+   option can be used for preserving TPM state. By default the value is ``no``.
+   This attribute only works with the ``emulator`` backend. The accepted values
+   are ``yes`` and ``no``. :since:`Since 7.0.0`
+
 ``encryption``
    The ``encryption`` element allows the state of a TPM emulator to be
    encrypted. The ``secret`` must reference a secret object that holds the

docs/schemas/domaincommon.rng

@@ -4766,6 +4766,14 @@
              <value>emulator</value>
           </attribute>
           <ref name="tpm-backend-emulator-encryption"/>
+          <optional>
+            <attribute name="persistent_state">
+              <choice>
+                <value>yes</value>
+                <value>no</value>
+              </choice>
+           </attribute>
+          </optional>
         </group>
       </choice>
       <choice>