lxd/device/disk: Ignore storageDrivers.ErrInUse error from pool.UnmountCustomVolume in postStop
lxd/device/disk: Ignore storageDrivers.ErrInUse error from pool.UnmountCustomVolume in postStop

This can occur when multiple running instances have the same disk attached and one of them is stopped.
The last one to stop will perform the actual unmount.

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

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