CP-6232: Don't restore the Qemu tail in xenopsd

We've removed a patch from our Xen patch queue which stopped xc_domain_restore
from restoring the device model record. Therefore, this will now be done by
xenguest and so we don't need to do it in xenopsd.

Aside: xc_domain_restore has been modified to understand suspend images created
before this change (i.e. when xenopsd orchestrated the device model dump).

Signed-off-by: Si Beaumont <simon.beaumont@citrix.com>
CP-6232: Use valid Qemu sig for future suspends

Signed-off-by: Si Beaumont <simon.beaumont@citrix.com>
CP-6323: Move Io to lib for use by both backends

Signed-off-by: Si Beaumont <simon.beaumont@citrix.com>
CP-6232: Write suspend img signature in xenopsd-xl

So that we can migrate from legacy xenopsd on a classic backend to xenopsd on
a xenlight backend, we need to be able to understand the save signature
("XenSavedDomain\n") that is at the start of the disk put there by the legacy
toolstack.

Since we would have to accomodate this and peeking is impracticle on pipes then
we will adopt this signature in the xenopsd-on-xenlight toolstack.

Note this is distinct from the Qemu save signature which has been discarded in
this work (see ef3bf4b and 9ccef3e).

Signed-off-by: Si Beaumont <simon.beaumont@citrix.com>
Add docs for suspend-image work

Signed-off-by: Si Beaumont <simon.beaumont@citrix.com>
Merge pull request #46 from simonjbeaumont/cp-6232

Support suspend/resume across different backends