lxd/migrate/instance: Fix snapshotToProtobuf to not use loop pointer for device name
lxd/migrate/instance: Fix snapshotToProtobuf to not use loop pointer for device name

This resulted in the device naming being sent incorrectly sometimes.

Fixes #8283

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

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