mapper: update to use new Devmapper interface
mapper: update to use new Devmapper interface

Note the function which converts an LV to a list of Devmapper.Target.ts
is hard-wired to use the pv.Pv.stored_device.

Signed-off-by: David Scott <63c9eb0ea83039690fefa11afe17873ba8278a56@citrix.com>

simonjbeaumont/mirage-block-volume,djs55/mirage-block-volume