Implementation of remote FS driver based on `rsync` for libvirt

libvirt driver uses ssh commands:
  - ssh touch,
  - ssh mkdir,
  - ssh rm,
  - rsync
 for communication with other nodes(for migration purposes).
This fact force us to use additional shell and it can cause security risk.
We can not avoid usage shell at all because of copying disk(uses rsync/scp call).
But it is possible to decrease the interface between nodes by using single rsync call instead of using ssh touch, ssh mkdir, ssh rm, rsync.
Usage rsync/scp commands only allows us to use secure shells like rssh.