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>
Merge pull request #1785 from Hao-Liu/add_swap

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