lxd/device/disk: Improvements in disk limits
lxd/device/disk: Improvements in disk limits

- Removes duplicated if diskPriority != "" check.
- Don't try and apply limits on stopped container during update.
- Fail if limits specified on device but device isn't mounted yet and is required (this avoids silently not setting limits).

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

stgraber/lxd,stgraber/lxd,lxc/lxd,Skarlso/lxd,lxc/lxd,hallyn/lxd,hallyn/lxd,Skarlso/lxd,lxc/lxd,mjeanson/lxd,hallyn/lxd,mjeanson/lxd,atxwebs/lxd,lxc/lxd,lxc/lxd,lxc/lxd,hallyn/lxd,hallyn/lxd,mjeanson/lxd,lxc/lxd,hallyn/lxd,stgraber/lxd,atxwebs/lxd,atxwebs/lxd,stgraber/lxd,stgraber/lxd,atxwebs/lxd,Skarlso/lxd,stgraber/lxd,stgraber/lxd,Skarlso/lxd