user: Restore debug usage message for '-d ?' in user mode emulation

user: Restore debug usage message for '-d ?' in user mode emulation

The code which prints the debug usage message on '-d ?' for *-user
has to come before the check for "not enough arguments", so that
"qemu-foo -d ?" prints the list of possible debug log items rather than
the generic usage message. (This was inadvertently broken in commit
c235d73.)

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>