usb_msd_init() calls qemu_opts_create() with a made-up ID and false
fail_if_exists.  If the ID already exists, it happily messes up those
options, then fails drive_new(), because the BlockDriverState with
that ID already exists, too.

Reproducer: -drive if=none,id=usb0,format=raw -usbdevice disk:tmp.qcow2

Pass true fail_if_exists to qemu_opts_create(), and if it fails, try
the next made-up ID.

The reproducer now succeeds, and creates an usb-storage device with ID
usb1.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>