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 the logic of parsing the sys-returned string in unit-testable code. The common code does conversion to int64 and change sectors to bytes.