migration: Make xbzrle_cache_size a migration parameter

Right now it is a variable in MigrationState instead of a
MigrationParameter.  The change allows to set it as the rest of the
Migration parameters, from the command line, with
query_migration_paramters, set_migrate_parameters, etc.

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