qemuDomainChangeNet: forbid changing portgroup

qemuDomainChangeNet: forbid changing portgroup

Changing the postgroup attribute caused unexpected behavior.
Although it can be implemented, it has a non-trivial solution.
No requirement or use has yet been found for implementing this
feature, so it has been disabled for hot-plug.

Resolves: https://issues.redhat.com/browse/RHEL-7299
Signed-off-by: Adam Julis <ajulis@redhat.com>
Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Michal Privoznik <mprivozn@redhat.com>