migration: Create options.c

We move there all capabilities helpers from migration.c.

Signed-off-by: Juan Quintela <quintela@redhat.com>
Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>

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Following David advise:
- looked through the history, capabilities are newer than 2012, so we
  can remove that bit of the header.
- This part is posterior to Anthony.
  Original Author is Orit. Once there,
  I put myself.  Peter Xu also did quite a bit of work here.
  Anyone else wants/needs to be there?  I didn't search too hard
  because nobody asked before to be added.

What do you think?