make cgpt deal with cciss devices

Hi,

when trying to install coreos on HP DL360 G5 servers, I discovered that cgpt couldn't manage cciss devices, because these devices are like:
/dev/cciss/c0d0....

cgpt, when reading /proc/partitions, tries to challenge the device name with a file in /sys/block. Unfortunately this last file path is different from the device name, because of the '/', which is converted to a '!'.

This PR adds the support to such devices (plus it removes a buffer overrun possibility).

Hope you will merge it,

--
rémi