qemuxml2argvtest: Don't leak TPM unix path

qemuxml2argvtest: Don't leak TPM unix path

When testing a domain XML with TPM we overwrite UNIX socket path
to mimic what qemuTPMEmulatorPrepareHost() is doing (because
*PrepareHost() functions are not called from the test). But we
are not doing it fully - we need to set the chardev's type too so
that virDomainTPMDefFree() can free the path.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Andrea Bolognani <abologna@redhat.com>