Allow disabling agent based file transfer;
Spice now supports copying files from client into guest using spice-vdagent (aka file transfer). By default this feature is enabled.
Users may want to disable it.
Result (if any):
A new disable-agent-file-transfer option for qemu's -spice argument. Spice file-transfer messages are dropped when qemu-kvm (and spice) are given this option.