USB: Improve usbdevice error messages
When an non-existent USB device is specified on the command line,
print "qemu: could not add USB device 'X'".
Likewise for the usb_{add,del} monitor commands.
Signed-off-by: Scott Tsai <scottt.tw@gmail.com>
Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>