stateful snapshots: rework behavior
stateful snapshots: rework behavior

* uidshift into the container's uidmap
* use forkmigrate so the monitor is not a child of lxd

Signed-off-by: Tycho Andersen <10c18fcb898ee6b17d6575d444b125d10b3f4d6a@canonical.com>

Skarlso/lxd,atxwebs/lxd,erikmack/lxd,ollie314/lxd,georghopp/lxd,yura-pakhuchiy/lxd,hallyn/lxd,ollie314/lxd,jsavikko/lxd,lxc/lxd,georghopp/lxd,chrisglass/lxd,mjeanson/lxd,Skarlso/lxd,stgraber/lxd,lxc/lxd,atxwebs/lxd,georghopp/lxd,akshaykarle/lxd,hallyn/lxd,ollie314/lxd,stgraber/lxd,techtonik/lxd,techtonik/lxd,lxc/lxd,pcdummy/lxd,yura-pakhuchiy/lxd,techtonik/lxd,jsavikko/lxd,chrisglass/lxd,lxc/lxd,mjeanson/lxd,stgraber/lxd,lxc/lxd,hallyn/lxd,lxc/lxd,erikmack/lxd,yura-pakhuchiy/lxd,pcdummy/lxd,akshaykarle/lxd,Skarlso/lxd,mjeanson/lxd,chrisglass/lxd,stgraber/lxd,Skarlso/lxd,stgraber/lxd,hallyn/lxd,hallyn/lxd,stgraber/lxd,hallyn/lxd,stgraber/lxd,atxwebs/lxd,pcdummy/lxd,erikmack/lxd,atxwebs/lxd,lxc/lxd,akshaykarle/lxd,jsavikko/lxd