xenserver: Correct location of xen-bugtool plugins.

xen-bugtool plugins were stored in "/etc/xensource/bugtool/bugtool-plugins/*",
but xen-bugtool will not traverse deeper than "/etc/xensource/bugtool/*".
This commit corrects the location and makes our plugins run.

signed-off-by: Justin Pettit <jpettit@nicira.com>
acked-by: Ben Pfaff <blp@nicira.com>