Add a disk map to machine info.
This is read once at start of cAdvisor. We can use this to report
machine state as well as return logical name for block devices in UI.

Signed-off-by: Rohit Jnagal <jnagal@google.com> (github: rjnagal)
Merge pull request #368 from rjnagal/diskinfo

Add a disk map to machine info.