Add bdev_sizes RPC call

The bdev_sizes multi-node RPC call returns the sizes of the requested
block devices on the desired nodes. Its intended use is to verify the
existence of a block device on a given node for shared block storage
support.

Block device paths are expected to lie under constants.BLOCKDEV_DIR
("/dev/disk" by default), where persistent symlinks for block devices
are assumed to exist.

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