target-i386: get/put MSR_TSC_AUX across reset and migration

There's one report of migration breaking due to missing MSR_TSC_AUX
save/restore.  Fix this by adding a new subsection that saves the state
of this MSR.

https://bugzilla.redhat.com/show_bug.cgi?id=1261797

Reported-by: Xiaoqing Wei <xwei@redhat.com>
Signed-off-by: Amit Shah <amit.shah@redhat.com>
CC: Paolo Bonzini <pbonzini@redhat.com>
CC: Juan Quintela <quintela@redhat.com>
CC: "Dr. David Alan Gilbert" <dgilbert@redhat.com>
CC: Marcelo Tosatti <mtosatti@redhat.com>
CC: Richard Henderson <rth@twiddle.net>
CC: Eduardo Habkost <ehabkost@redhat.com>
Reviewed-by: Eduardo Habkost <ehabkost@redhat.com>
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>
