Guest storage: Fix attaching type judgement

When we only supportted cdrom attachment, we check if a cdrom is
a valid block device by reading /proc info.
Now we added disk attachment support, when a file is neither regular file
nor url, we need to see if it is a block device according to its stats.

Signed-off-by: Royce Lv <lvroyce@linux.vnet.ibm.com>