migration: Use proper types in json

We use int for everything (int64_t), and then we check that value is
between 0 and 255.  Change it to the valid types.

This change only happens for HMP.  QMP always use bytes and similar.

Signed-off-by: Juan Quintela <quintela@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>