migration: move only_migratable to MigrationState

One less global variable, and it does only matter with migration.

We keep the old "--only-migratable" option, but also now we support:

  -global migration.only-migratable=true

Currently still keep the old interface.

Hmm, now vl.c has no way to access migrate_get_current(). Export a
function for it to setup only_migratable.

Reviewed-by: Juan Quintela <quintela@redhat.com>
Signed-off-by: Peter Xu <peterx@redhat.com>
Message-Id: <1498536619-14548-7-git-send-email-peterx@redhat.com>
Signed-off-by: Juan Quintela <quintela@redhat.com>