qemu: mock swtpm initialization in tests

The domain capabilities won't report TPM support unless SWTPM can be
initialized. To avoid relying on the swtpm install in the host, mock
the entire initialization method, since all it needs todo is return
a non-error value.

Reviewed-by: Ján Tomko <jtomko@redhat.com>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>

tests/domaincapsdata/qemu_4.2.0.x86_64.xml

@@ -210,6 +210,7 @@
       </enum>
       <enum name='backendModel'>
         <value>passthrough</value>
+        <value>emulator</value>
       </enum>
     </tpm>
   </devices>