storage: add ploop volume type

storage: add ploop volume type

Ploop image consists of directory with two files: ploop image itself,
called root.hds and DiskDescriptor.xml that contains information about
ploop device: https://openvz.org/Ploop/format.
Such volume are difficult to manipulate in terms of existing volume types
because they are neither a single files nor a directory.
This patch introduces new volume type - ploop. This volume type is used
by ploop volume's exclusively.

Signed-off-by: Olga Krishtal <okrishtal@virtuozzo.com>
Signed-off-by: Ján Tomko <jtomko@redhat.com>