tests/migration: Introduce dirty-ring-size option into guestperf

Dirty ring size configuration is not supported by guestperf tool.

Introduce dirty-ring-size (ranges in [1024, 65536]) option so
developers can play with dirty-ring and dirty-limit feature easier.

To set dirty ring size with 4096 during migration test:
$ ./tests/migration/guestperf.py --dirty-ring-size 4096 xxx

Signed-off-by: Hyman Huang <yong.huang@smartx.com>
Reviewed-by: Fabiano Rosas <farosas@suse.de>
Signed-off-by: Juan Quintela <quintela@redhat.com>
Message-ID: <8a388cec5c1f73a34d42515bbc43837e97ee3839.1698847223.git.yong.huang@smartx.com>