virt: Remove useless wrap of at-detach disk.

Signed-off-by: Yu Mingfei <yumingfei@cn.fujitsu.com>
virttest.utils_libguestfs: Add functions for Guestfish

Signed-off-by: Yu Mingfei <yumingfei@cn.fujitsu.com>
libguestfs.tests: Add testcases for guestfish partition operations.

Prepare: Attach a new disk to guest, and assign to vdb.
Testcases: Operate vdb with partition operations: part-*.
Signed-off-by: Yu Mingfei <yumingfei@cn.fujitsu.com>