Mounted partitions space usage metrics per container

This computes the space usage for mounted partitions. It takes in a list of mounted partitions from containerHints
and computes the device's disk usage(so each mount must be a separate partition). This is useful for users who
mount partitions on containers and store most of the container's persistent data on those partitions.
shouldn't we do the deviceSet check before the getVfsStats so we don't even make the syscall in that case?