virt: Add virt_cmd function in utils_libguestfs module

Signed-off-by: Li Yang <liyang.fnst@cn.fujitsu.com>
virt-libguestfs: Add test for virt-sysprep command

1. Clone a guest and login this guest to do some actions
2. Execute virt-sysprep to reset or unconfigure a virtual
   machine so clones can be made
3. Confirm reset result.

Signed-off-by: Li Yang <liyang.fnst@cn.fujitsu.com>