lxd/instance/post: Rework instancePostClusteringMigrateWithCeph to support VM stateful migration
lxd/instance/post: Rework instancePostClusteringMigrateWithCeph to support VM stateful migration

Also supports starting and stopping instance when performing stateles migration.
Also various improvements to errors and comments.

Signed-off-by: Thomas Parrott <6b778ce645fb0e3dde76d79eccad490955b1ae74@canonical.com>

lxc/lxd,lxc/lxd,hallyn/lxd,hallyn/lxd,lxc/lxd,hallyn/lxd,stgraber/lxd,lxc/lxd,hallyn/lxd,lxc/lxd,hallyn/lxd,stgraber/lxd,lxc/lxd,stgraber/lxd,stgraber/lxd,lxc/lxd,hallyn/lxd,stgraber/lxd,stgraber/lxd,stgraber/lxd