tools: remus: support DRBD disk backends

tools: remus: support DRBD disk backends

DRBD disk backends can be used instead of tapdisk backends for Remus.
This requires a Remus style disk replication protocol (asynchronous
replication with output buffering at backup), that is not available in
standard DRBD code. A modified version that supports this new replication
protocol is available from git://aramis.nss.cs.ubc.ca/drbd-8.3-remus

Use of DRBD disk backends provides a means for efficient
resynchronization of data after the crashed machine comes back
online. Since DRBD allows for online resynchronization, a DRBD backed
Remus VM does not have to be stopped or shutdown while the disks are
resynchronizing. Once resynchronization is complete, Remus can be
started at will.

Signed-off-by: Shriram Rajagopalan <rshriram@cs.ubc.ca>
Committed-by: Ian Jackson <ian.jackson@eu.citrix.com>