xl/remus: cmdline switch to explicitly enable unsafe configurations

xl/remus: cmdline switch to explicitly enable unsafe configurations

By default, network buffering and disk replication are enabled;
checkpoints are replicated to another standby VM.

This patch allows the user to disable any of these features by
explicitly specifying a 'run in unsafe mode' switch when invoking
the 'xl remus' command.  While running Remus in an unsafe mode
makes little sense under normal circumstances, it is useful to be
able to disable one or more features mentioned above for
testing/debugging/profiling purposes.

Unless this option is enabled, it will not be possible to
replicate memory checkpoints to /dev/null (blackhole replication),
disable network buffering or disk replication.

As a starter, the use of blackhole replication now requires that
the unsafe mode be enabled. Subsequent patches will add support
for disabling network buffering and disk replication in a similar
manner.

Signed-off-by: Yang Hongyang <yanghy@cn.fujitsu.com>
Signed-off-by: Shriram Rajagopalan <rshriram@cs.ubc.ca>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>
Signed-off-by: Ian Jackson <Ian.Jackson@eu.citrix.com>
Reviewed-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>