vl.c: Error locations for options using add_device_config()

vl.c: Error locations for options using add_device_config()

These are -bt, -serial, -virtcon, -parallel, -debugcon, -usbdevice.
Improves messages emitted via proper error reporting interfaces.  For
instance:

    $ qemu-system-x86_64 -nodefaults -S -usb -usbdevice net:vlan=xxx
    qemu-system-x86_64: Parameter 'vlan' expects a number

becomes:

    qemu-system-x86_64: -usbdevice net:vlan=xxx: Parameter 'vlan' expects a number

Many more remain unimproved, because they're fprintf()ed.  The next
few commits will take care of that.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>