lxd/storage/drivers/driver/dir/volumes: Don't allow creating a volume that already exists on disk
lxd/storage/drivers/driver/dir/volumes: Don't allow creating a volume that already exists on disk

Otherwise if something goes wrong later we may end up deleting the existing directory during
the revert stage, thinking we had just created it, but actually it may contain existing files.

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

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