CLOUDSTACK-7193: handle domain ID being an int

Recent versions of libvirt (at least 0.9.8) will return an int when
queried for the ID of a domain, not a string. This breaks some parts of
the `security_group.py` script which expects a string containing an
int. Notably, this breaks the part handling VM reboots which is
therefore not executed.

Signed-off-by: Vincent Bernat <Vincent.Bernat@exoscale.ch>