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)
Long time ago, in the first version of this PR, it was a struct :) We moved it to string because one of the operation was not supported with a struct key. I don't remember what it was - maybe some kind of copy. Need to look it up.