[PATCH 0 of 5] Make checkpointing backwards compatible with older kernels

[Brendan Cully <brendan@cs.ubc.ca>]

This set of patches makes checkpointing backwards compatible with
older kernels. Checkpointable kernels now create a xenstore entry
called "control/feature-resumable" which xc save -c checks before
attempting to checkpoint a domain.

It also writes a new SIF_RESUMABLE flag into the suspend record,
which xc_linux_restore uses to decide whether it is safe to set the
hypercall return value. Although pre-checkpoint linux images ignore
the return value, I've heard that solaris bails out if the result is
anything but 0. Another possibly simpler strategy would be to flip the
return values, so 1 means the call is resuming in the source domain
and 0 means it is in a new domain and must reconnect. This is what
older kernels (that are only ever restored in a new domain) expect
anyway.