migration: do not allow live-migration with clipboard=vnc

migration: do not allow live-migration with clipboard=vnc

It is not yet supported for QEMU's vdagent device which is used for
the VNC clipboard.

The migration precondition API call will now treat the VNC clipboard
as a local resource. Thus the GUI blocks migration and shows:
"Can't migrate VM with local resources: clipboard=vnc"

QemuMigrate's prepare function will also abort live migration early
when using the VNC clipboard.

Signed-off-by: Markus Frank <m.frank@proxmox.com>
[FE: adapt commit message a bit]
Signed-off-by: Fiona Ebner <f.ebner@proxmox.com>