Xen supports on_crash actions coredump-{destroy,restart}.  libvirt
cannot parse config returned by xend that contains either of these
actions

xen52 # xm li -l test | grep on_crash
    (on_crash coredump-restart)
xen52 # virsh dumpxml test
error: internal error unknown lifecycle type coredump-restart

This patch adds a new virDomainLifecycleCrash enum and appends
the new options to existing destroy, restart, preserve, and
rename-restart options.