* Unifies the declaration of callbacks which manage driver/client
shared dataspaces
* Move the Linux driver-specific callback implementation to the
lx_emul library from the PC's USB host driver
Fixgenodelabs/genode#4439
Adds a function to the USB part of the Genode's C API, to enable
usb_host drivers to acknowledge USB request in client's packet buffer
although they are not assigned to an USB device. The requests are
marked with a "no device" error.
This commit fixes a regression originally solved in genodelabs/genode#4149
Ref genodelabs/genode#4416
