Split RemoteFS drivers code to FS handlers and Volume Format handlers

RemoteFS drivers combine 2 logical tasks. The first one is how to mount a filesystem and select proper share for a new or existing volume. The second one: how to deal with an image files in given directory (mount point) (create, delete, create snapshot e.t.c.).

The first part is different for each volume driver. The second - the same for all volume drivers, but it depends on selected volume format: you can create qcow2 file on NFS or smbfs with the same code.

Since there are several volume formats (raw, qcow2, vhd and possibly some others), I propose to move the code, which works with image to separate classes, 'VolumeFormat' handlers.

There are two advantages of this approach: all remotefs volume driver will support all volume formats and snapshots and there will be no duplicated code.