libvirt_vm: Add kernel setter, swap part/file, disk attach/detach

1) Add a function set_kernel_param() to change or remove a kernel
parameter from VM's grub configation file.
2) Add functions to add and activate a swap partition or a swap file.
3) Add functions to attach or detach a image file as a disk.

Signed-off-by: Hao Liu <hliu@redhat.com>
The output of ```df /swapfile``` may not stable all the time, if we use VG, the output may like:
```
# df /swapfile 
Filesystem           1K-blocks     Used Available Use% Mounted on
/dev/mapper/VolGroup-LogVol_root
                     422265368 14605436 386203448   4% /
```