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)
WDYT of having a struct for Major and Minor. In Go we can use that as a string for a map.