Add 4 cases to test following guestfish commands

blockdev-getbsz
blockdev-getsize64
blockdev-getss
blockdev-getsz
guestfish_block_dev: fix a minior issue

1.Modify the way of parsing the string, it can be sutiable for more
situtation
2.Remove the fs type from cfg file, it's useless to block_dev test
Merge pull request #98 from vanbogithub/bfan-dev

Add 4 cases to test following guestfish commands