xen/pvhvm: Initialize xen panic handler for PVHVM guests

kernel use callback linked in panic_notifier_list to notice others when panic
happens.
NORET_TYPE void panic(const char * fmt, ...){
    ...
    atomic_notifier_call_chain(&panic_notifier_list, 0, buf);
}
When Xen becomes aware of this, it will call xen_reboot(SHUTDOWN_crash) to
send out an event with reason code - SHUTDOWN_crash.

xen_panic_handler_init() is defined to register on panic_notifier_list but
we only call it in xen_arch_setup which only be called by PV, this patch is
necessary for PVHVM.

Without this patch, setting 'on_crash=coredump-restart' in PVHVM guest config
file won't lead a vmcore to be generate when the guest panics. It can be
reproduced with 'echo c > /proc/sysrq-trigger'.

Signed-off-by: Vaughan Cao <vaughan.cao@oracle.com>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Acked-by: Joe Jin <joe.jin@oracle.com>