x86: Raise vsyscall priority on hotplug notifier chain

KVM need vsyscall_init() to initialize MSR_TSC_AUX before it read the value.
Per Avi's suggestion, this patch raised vsyscall priority on hotplug notifier
chain, to 30.

CC: Ingo Molnar <mingo@elte.hu>
CC: linux-kernel@vger.kernel.org
Signed-off-by: Sheng Yang <sheng@linux.intel.com>
Signed-off-by: Avi Kivity <avi@redhat.com>