libguestfs.test: Fix guestfs_add for new libguestfs version.

In new libguestfs version, all disks will be recognised with /dev/sdx.
But virtio disks of vm will be /dev/vdx in vm.So change gotten
/dev/sdx in guestfish to /dev/vdx if it is necessary.
Signed-off-by: NYu Mingfei <yumingfei@cn.fujitsu.com>