Using libvirt to do live migration over RDMA via IPv6 address failed.

For example:
    rhel73_host1_guest1 qemu+ssh://[deba::2222]/system --verbose
root@deba::2222's password:
error: internal error: unable to execute QEMU command 'migrate': RDMA
ERROR: could not rdma_getaddrinfo address deba

As we can see, the IPv6 address used by rdma_getaddrinfo() has only
"deba" part because we didn't properly enclose the IPv6 address in []
and passed rdma:deba::2222:49152 as the migration URI in
qemuMonitorMigrateToHost.
Signed-off-by: NDavid Dai <zdai@linux.vnet.ibm.com>