lxd/storage_dir: rsync + freeze + rsync unfreeze
lxd/storage_dir: rsync + freeze + rsync unfreeze

This tries to ensure that dir backed containers are in a consistent state when
snapshotted.

Signed-off-by: Christian Brauner <48455ab3070520a2d174545c7239d6d0fabd9a83@canonical.com>

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