x86/xen: Zero MSR_IA32_SPEC_CTRL before suspend

Older Xen versions (4.5 and before) might have problems migrating pv
guests with MSR_IA32_SPEC_CTRL having a non-zero value. So before
suspending zero that MSR and restore it after being resumed.

Signed-off-by: Juergen Gross <jgross@suse.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
Cc: stable@vger.kernel.org
Cc: xen-devel@lists.xenproject.org
Cc: boris.ostrovsky@oracle.com
Link: https://lkml.kernel.org/r/20180226140818.4849-1-jgross@suse.com