Introduce migration precopy policy

Introduce migration precopy policy

This Patch allows a migration precopy policy to be specified.

The precopy phase of the xc_domain_save() live migration algorithm has
historically been implemented to run until either a) (almost) no pages
are dirty or b) some fixed, hard-coded maximum number of precopy
iterations has been exceeded.  This policy and its implementation are
less than ideal for a few reasons:
- the logic of the policy is intertwined with the control flow of the
  mechanism of the precopy stage
- it can't take into account facts external to the immediate
  migration context, such external state transfer state, interactive
  user input, or the passage of wall-clock time.
- it does not permit the user to change their mind, over time, about
  what to do at the end of the precopy (they get an unconditional
  transition into the stop-and-copy phase of the migration)

To permit callers to implement arbitrary higher-level policies governing
when the live migration precopy phase should end, and what should be
done next:
- add a precopy_policy() callback to the xc_domain_save() user-supplied
  callbacks
- during the precopy phase of live migrations, consult this policy after
  each batch of pages transmitted and take the dictated action, which
  may be to a) abort the migration entirely, b) continue with the
  precopy, or c) proceed to the stop-and-copy phase.
- provide an implementation of the old policy, used when
  precopy_policy callback  is not provided.

Signed-off-by: Jennifer Herbert <Jennifer.Herbert@citrix.com>
Signed-off-by: Joshua Otto <jtotto@uwaterloo.ca>
Reviewed-by: Paul Durrant <paul.durrant@citrix.com>
Acked-by: Wei Liu <wei.liu2@citrix.com>