partitionedfs: save the mapper device path

partitionedfs: save the mapper device path

For some legacy reasons related to grub, which we do not use, MIC creates
/dev/loopXpY files which are symlinks to the real /dev/mapper/loopXpY device
node files. Then MIC saves the /dev/loopXpY path in the internal data
structure, and drps the mapper device path.

This patch makes MIC also save the /dev/mapper/loopXpY in the internal data
structures for firther use. We'll use it later for creating a copy of device
nodes in the target image. Thede device nodes will be later used by installer
framework plugins.

Change-Id: Ia928669ffd40a8fbda472d2f9d9890053aa94f67
Signed-off-by: Artem Bityutskiy <artem.bityutskiy@intel.com>