Legacy PCI device assignment has been removed from Linux in 4.12,
and had been deprecated 2 years ago there.  We can remove it from
QEMU as well.

The ROM loading code was shared with Xen PCI passthrough, so move
it to hw/xen.

Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>