lxd/device/disk: Only validate disk source pool when an actual instance is set
lxd/device/disk: Only validate disk source pool when an actual instance is set

Allows for migration of instances who have snapshots with invalid devices.

Fixes #8187

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

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