Clipboard sharing with `qemu_vdagent` does not work with SDL backend
Description of problem:
Clipboard sharing doesn't work: qemu does not send clipboard-grab messages when selecting on the host, nor does it respond to clipboard-grab messages from the guest.
Steps to reproduce:
1. Start QEMU with `qemu_vdagent` and `-display sdl`
2. Try to copy on the host or the guest
3. Observe that the clipboard is not shared
Additional information:
It appears as though `vdagent_clipboard_notify` function is not called.

Logs: 

With SDL:
```
vdagent_open 
vdagent_recv_chunk size 28
vdagent_recv_msg msg announce-capabilities, size 8
vdagent_peer_cap cap mouse-state
vdagent_peer_cap cap monitors-config
vdagent_peer_cap cap reply
vdagent_peer_cap cap clipboard-by-demand
vdagent_peer_cap cap clipboard-selection
vdagent_peer_cap cap sparse-monitors-config
vdagent_peer_cap cap guest-lineend-lf
vdagent_peer_cap cap max-clipboard
vdagent_peer_cap cap audio-volume-sync
vdagent_send msg announce-capabilities
# tried to copy on host -- nothing happens here.
# trying to copy on guest:
vdagent_recv_chunk size 28
vdagent_recv_msg msg clipboard-grab, size 8
vdagent_cb_grab_selection selection clipboard
vdagent_cb_grab_type type text
# no response sent
```
With GTK:
```
vdagent_open 
vdagent_recv_chunk size 28
vdagent_recv_msg msg announce-capabilities, size 8
vdagent_peer_cap cap mouse-state
vdagent_peer_cap cap monitors-config
vdagent_peer_cap cap reply
vdagent_peer_cap cap clipboard-by-demand
vdagent_peer_cap cap clipboard-selection
vdagent_peer_cap cap sparse-monitors-config
vdagent_peer_cap cap guest-lineend-lf
vdagent_peer_cap cap max-clipboard
vdagent_peer_cap cap audio-volume-sync
vdagent_send msg announce-capabilities
# trying to copy on host:
vdagent_send msg clipboard-grab
vdagent_recv_chunk size 28
vdagent_recv_msg msg clipboard-request, size 8
vdagent_send msg clipboard
vdagent_recv_chunk size 28
# trying to copy on guest:
vdagent_recv_msg msg clipboard-grab, size 8
vdagent_cb_grab_selection selection clipboard
vdagent_cb_grab_type type text
vdagent_send msg clipboard-request
vdagent_recv_chunk size 29
vdagent_recv_msg msg clipboard, size 9
```