Shared block storage support

This patch introduces basic shared block storage support.

It introduces a new storage backend, bdev.PersistentBlockDevice, to
use as a backend for shared block storage. The new bdev requires a new
BLOCKDEV_DRIVER_MANUAL constant with the value "manual" and uses it as
the first part of the block device unique_id.

A new disk template, DT_BLOCK is introduced as well and added to
DTS_EXT_MIRROR and DTS_MAY_ADOPT. Also added DTS_MUST_ADOPT constant
and use it to check for the presence of the adopt keyword during LU
invocation. We enforce the /dev/disk limitation upon adoption, but we
allow block devices to reside anywhere under /dev.

This is very basic support and includes no storage manipulation (provisioning,
resizing, renaming) which will have to be implemented through a "driver"
framework.

Signed-off-by: Apollon Oikonomopoulos <apollon@noc.grnet.gr>
[iustin@google.com: slight changes to bdev.py]
Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>