vl.c: Fix broken -usb option

vl.c: Fix broken -usb option

Commit 094b287f0b accidentally broke the "-usb" command line
option, so it would have no effect if the user had not specified
any machine options at that point. (the return value from
'qemu_opts_find(qemu_find_opts("machine"), 0);' is NULL if there
are no user specified options, so it is only to be used for
looking up an option, not when trying to set one.) Similarly,
would '-usbdevice' no longer cause USB to default to enabled.

Fix this regression by using the same style of code for forcing
the usb=on machine option that we use for other aliases such as
'-enable-kvm'.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>