Randomize choose_pbd_for_sr
Randomize choose_pbd_for_sr

... hence randomize forward_sr_op and forward_vdi_op as well.

Signed-off-by: Zheng Li <fff69bf00b8ec5b6098a1768eaaaa75f40b8795d@eu.citrix.com>

robhoes/squeezed,simonjbeaumont/xcp-rrdd,sharady/xcp-networkd,simonjbeaumont/xcp-rrdd,djs55/xcp-rrdd,djs55/squeezed,djs55/xcp-networkd,koushikcgit/xcp-rrdd,koushikcgit/xcp-networkd,koushikcgit/xcp-rrdd,koushikcgit/xcp-rrdd,sharady/xcp-networkd,djs55/xcp-networkd,johnelse/xcp-rrdd,johnelse/xcp-rrdd,djs55/xcp-rrdd,koushikcgit/xcp-networkd