libguestfs.tests: Del blocksize case from guestfs_block_operations.

According to manpage, blockdev-setbsz has been deprecated:
This call does nothing and has never done anything because of
a bug in blockdev.  Do not use it.

Signed-off-by: Yu Mingfei <yumingfei@cn.fujitsu.com>
Merge pull request #306 from yumingfei/del_gf_setbsz

libguestfs.tests: Del blocksize case from guestfs_block_operations.