[Crash-utility] A few more xen hypervisor fixes/updates

Hello Oda-san, Here are a couple more xen hypervisor specific patches that I plan to apply to the next release. The first patch is for the "bt" command when run on the xen hypervisor. The patch will reject "bt -o", "bt -O", "bt -e" and "bt -E" with "option not supported" messages. Additionally, it fixes "bt -R" so that it works correctly, and does not cause a segmentation violation. The second patch removes "foreach" from the list of xen hypervisor commands. It could never have possibly worked, and if you attempted to actually use it, it would silently fail. Unless you have any objections, they are queued for the next release. Thanks, Dave