In some BSD systems RDMA migration is possible while
the pvrdma device can't be used because the mremap system call
is missing.

Reported-by: Rebecca Cran <rebecca@bluestop.org>
Signed-off-by: Marcel Apfelbaum <marcel.apfelbaum@gmail.com>
Message-Id: <20180816151637.24553-1-marcel.apfelbaum@gmail.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>