libqos: Add migration helpers

libqos: Add migration helpers

libqos.c:
    -set_context for addressing which commands go where
    -migrate performs the actual migration

malloc.c:
    - Structure of the allocator is adjusted slightly with
      a second-tier malloc to make swapping around the allocators
      easy when we "migrate" the lists from the source to the destination.

Signed-off-by: John Snow <jsnow@redhat.com>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Message-id: 1430417242-11859-4-git-send-email-jsnow@redhat.com