rdma: Fix qemu crash when IPv6 address is used for migration

Qemu crashes when IPv6 address is specified for migration and access
to any RDMA uverbs device available on the system is blocked using cgroups.
Fix the crash by checking the return value of ibv_open_device routine.

Signed-off-by: Meghana Cheripady <meghana.cheripady@avagotech.com>
Signed-off-by: Padmanabh Ratnakar <padmanabh.ratnakar@avagotech.com>
Signed-off-by: Juan Quintela <quintela@redhat.com>
