conf: Move _virDomainTPMDef::version into _virDomainTPMDef::data::emulator

conf: Move _virDomainTPMDef::version into _virDomainTPMDef::data::emulator

The _virDomainTPMDef structure has 'version' member, which is a
bit misplaced. It's only emulator type of TPM that can have a
version, even our documentation says so:

``version``
   The ``version`` attribute indicates the version of the TPM. This attribute
   only works with the ``emulator`` backend. The following versions are
   supported:

Therefore, move the member into that part of union that's
covering emulated TPM devices.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Peter Krempa <pkrempa@redhat.com>