Introduce an usb device flag instead, set it when usb-host looks at the
device descriptors anyway.  Also set it for emulated storage devices,
for consistency.  Add an inline helper function to check the flag.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Acked-by: David Gibson <david@gibson.dropbear.id.au>
Reviewed-by: Jose R. Ziviani <jziviani@suse.de>
Message-Id: <20210624103836.2382472-32-kraxel@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>