lxd/device/disk: Change mountPoolVolume to return a revert function
lxd/device/disk: Change mountPoolVolume to return a revert function

Rather than accept a reverter. The reason for this is that although covnenient, passing a reverter into a function is not ideal because it
does not allow local revertion using the same reverter, and requires the caller to pass in a reverter even if they dont care about reversion.

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

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