volume: add the extendVolumeSize method

This patch adds the extendVolumeSize SPM method to implement the volume
size extension (disk resize).

 extendVolumeSize(spUUID, sdUUID, imgUUID, volUUID, newSize)

The argument spUUID, sdUUID, imgUUID, volUUID indentify the volume to
extend, newSize is the new size of the volume in bytes.

Change-Id: I817be851d7fbe080adf4da297b526837e244cec6
Signed-off-by: Federico Simoncelli <fsimonce@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/14590
Reviewed-by: Sergey Gotliv <sgotliv@redhat.com>
Tested-by: Sergey Gotliv <sgotliv@redhat.com>
Reviewed-by: Allon Mureinik <amureini@redhat.com>
Reviewed-by: Ayal Baron <abaron@redhat.com>