qemuhotplugtest: Don't overwrite vm->def->id in testQemuHotplugCheckResult()

qemuhotplugtest: Don't overwrite vm->def->id in testQemuHotplugCheckResult()

This is a leftover from v2.0.0-rc1~300. In v1.2.12-rc1~43 we've
introduced a code that explicitly sets vm->def->id to -1 to force
generation of inactive XML. But this was removed in the later
commit, which forgot to remove the restoration of the original
dom ID.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Kristina Hanicova <khanicov@redhat.com>