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)
The next step is to reconcile this with the FS info we already have. Note that these are real block devices - like sda and not the mounted partitions (sda1).