check the return value of the function it calls and error if it's non-0
Fixup qemu_rdma_init_one_block that is the only current caller,
  and rdma_add_block the only function it calls using it.

Pass the name of the ramblock to the function; helps in debugging.
Signed-off-by: NDr. David Alan Gilbert <dgilbert@redhat.com>
Reviewed-by: NDavid Gibson <david@gibson.dropbear.id.au>
Reviewed-by: NAmit Shah <amit.shah@redhat.com>
Reviewed-by: NMichael R. Hines <mrhines@us.ibm.com>
Reviewed-by: NJuan Quintela <quintela@redhat.com>
Signed-off-by: NJuan Quintela <quintela@redhat.com>