qemu: Fill in model of 'usb' disks to preserve ABI compatibility

qemu: Fill in model of 'usb' disks to preserve ABI compatibility

While 'usb-bot' and 'usb-storage' are ABI and migration compatible for
disks it's not the case for cdroms. When migrating from a new config
using 'usb-bot' to an older daemon which would use 'usb-storage' the
guest os will get I/O errors.

Thus we must properly fill in models for 'usb' disks so that cdroms can
be handled properly.

When parsing XML fill in the models and drop the appropriate models when
formatting migratable XML.

The logic is explained in comments in the code.

Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Jiri Denemark <jdenemar@redhat.com>