hw/rdma: Change host_virt to void *

To avoid compilation warnings on 32-bit machines:
rdma_backend.c: In function 'rdma_backend_create_mr':
rdma_backend.c:409:37: error: cast to pointer from integer of different
size [-Werror=int-to-pointer-cast]
	mr->ibmr = ibv_reg_mr(pd->ibpd, (void *)addr, length, access);

Reported-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Yuval Shaia <yuval.shaia@oracle.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Tested-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Message-Id: <20180322095220.9976-2-yuval.shaia@oracle.com>
Signed-off-by: Marcel Apfelbaum <marcel@redhat.com>