Remove some warnings from Clang static analyzer

qmp-port.c: warning: Although the value stored to 'node' is used in the enclosing expression, the value is never actually read from 'node'

usb-backend.c: warning: Value stored to 'done' is never read warning: Use of memory after it is freed

usb-device-cd.c: warning: Value stored to 'error' is never read

Signed-off-by: Frediano Ziglio freddy77@gmail.com