lxd/device/device/utils/disk: Use storageDrivers.TryUnmount without MNT_DETACH in DiskMountClear
lxd/device/device/utils/disk: Use storageDrivers.TryUnmount without MNT_DETACH in DiskMountClear

So that we ensure the mount is cleaned up or error trying, rather than continue in an unknown state.

This is important when shutting down VMs on an LVM backend so that we can reliably know when the mount
has been cleared so we can deactive the logical volume.

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

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