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)
I kept it same as what actual os interface returns, so we end up running all parsing code through unit tests. string cast isn't a big deal though.