libvirt.tests: Add filter for migrate speed with bz1083483.

Signed-off-by: John Ferlan <jferlan@redhat.com>
libvirt.tests: Wrap set_get_speed operations to single function.

Signed-off-by: Yu Mingfei <yumingfei@cn.fujitsu.com>
tests.migrate_set_get_speed: Remove useless parameters.

Signed-off-by: Yu Mingfei <yumingfei@cn.fujitsu.com>
Merge pull request #184 from yumingfei/migrate_speed_enhancements

libvirt.tests: Migrate speed enhancements