#11 Separate I/O stats for multiple ubd's

[Chia Tee Kiah]

This patch separates out the I/O accounting for
"/dev/ubd/" devices in "/proc/stat", in cases where
multiple such devices are in use; for example, instead
of just

disk_io: (98,0):(100,50,1000,50,500)

it is now possible to obtain

disk_io: (98,0):(40,10,700,30,300)
(98,1):(60,40,300,20,200)

[Chia Tee Kiah]