multifd: Add multifd-zstd-level parameter

This parameter specifies the zstd compression level. The next patch
will put it to use.

Signed-off-by: Juan Quintela <quintela@redhat.com>
Reviewed-by: Peter Xu <peterx@redhat.com>
Acked-by: Markus Armbruster <armbru@redhat.com>
4 files changed