libvirt: Add virsh migrate-set/get speed command test

New testcase: virsh_migrate_set_get_speed.cfg/py
virsh.py: make migrate_getspeed() return CmdResult object
          rather than only result string.
Merge pull request #1302 from Guannan-Ren/migrate_get_set_speed

libvirt: Add virsh migrate-set/get speed command test