migration: implement io_set_aio_fd_handler function for RDMA QIOChannel

if qio_channel_rdma_readv return QIO_CHANNEL_ERR_BLOCK, the destination qemu
crash.

The backtrace is:
(gdb) bt
    #0  0x0000000000000000 in ?? ()
    #1  0x00000000008db50e in qio_channel_set_aio_fd_handler (ioc=0x38111e0, ctx=0x3726080,
        io_read=0x8db841 <qio_channel_restart_read>, io_write=0x0, opaque=0x38111e0) at io/channel.c:
    #2  0x00000000008db952 in qio_channel_set_aio_fd_handlers (ioc=0x38111e0) at io/channel.c:438
    #3  0x00000000008dbab4 in qio_channel_yield (ioc=0x38111e0, condition=G_IO_IN) at io/channel.c:47
    #4  0x00000000007a870b in channel_get_buffer (opaque=0x38111e0, buf=0x440c038 "", pos=0, size=327
        at migration/qemu-file-channel.c:83
    #5  0x00000000007a70f6 in qemu_fill_buffer (f=0x440c000) at migration/qemu-file.c:299
    #6  0x00000000007a79d0 in qemu_peek_byte (f=0x440c000, offset=0) at migration/qemu-file.c:562
    #7  0x00000000007a7a22 in qemu_get_byte (f=0x440c000) at migration/qemu-file.c:575
    #8  0x00000000007a7c78 in qemu_get_be32 (f=0x440c000) at migration/qemu-file.c:655
    #9  0x00000000007a0508 in qemu_loadvm_state (f=0x440c000) at migration/savevm.c:2126
    #10 0x0000000000794141 in process_incoming_migration_co (opaque=0x0) at migration/migration.c:366
    qemu#11 0x000000000095c598 in coroutine_trampoline (i0=84033984, i1=0) at util/coroutine-ucontext.c:1
    qemu#12 0x00007f9c0db56d40 in ?? () from /lib64/libc.so.6
    qemu#13 0x00007f96fe858760 in ?? ()
    qemu#14 0x0000000000000000 in ?? ()

RDMA QIOChannel not implement io_set_aio_fd_handler. so
qio_channel_set_aio_fd_handler will access NULL pointer.

Signed-off-by: Lidong Chen <lidongchen@tencent.com>
Reviewed-by: Juan Quintela <quintela@redhat.com>
Signed-off-by: Juan Quintela <quintela@redhat.com>

migration/rdma.c

@@ -2963,6 +2963,21 @@ static GSource *qio_channel_rdma_create_watch(QIOChannel *ioc,
     return source;
 }
 
+static void qio_channel_rdma_set_aio_fd_handler(QIOChannel *ioc,
+                                                  AioContext *ctx,
+                                                  IOHandler *io_read,
+                                                  IOHandler *io_write,
+                                                  void *opaque)
+{
+    QIOChannelRDMA *rioc = QIO_CHANNEL_RDMA(ioc);
+    if (io_read) {
+        aio_set_fd_handler(ctx, rioc->rdmain->comp_channel->fd,
+                           false, io_read, io_write, NULL, opaque);
+    } else {
+        aio_set_fd_handler(ctx, rioc->rdmaout->comp_channel->fd,
+                           false, io_read, io_write, NULL, opaque);
+    }
+}
 
 static int qio_channel_rdma_close(QIOChannel *ioc,
                                   Error **errp)
@@ -3822,6 +3837,7 @@ static void qio_channel_rdma_class_init(ObjectClass *klass,
     ioc_klass->io_set_blocking = qio_channel_rdma_set_blocking;
     ioc_klass->io_close = qio_channel_rdma_close;
     ioc_klass->io_create_watch = qio_channel_rdma_create_watch;
+    ioc_klass->io_set_aio_fd_handler = qio_channel_rdma_set_aio_fd_handler;
 }
 
 static const TypeInfo qio_channel_rdma_info = {