lxd/device/disk: Replaces call to StorageVolumeMount with functions on disk device
lxd/device/disk: Replaces call to StorageVolumeMount with functions on disk device

StorageVolumeMount was only used by disk device.

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

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