kvm/x86: Reorg stimer_expiration() to better control timer restart

Split stimer_expiration() into two parts - timer expiration message
sending and timer restart/cleanup based on timer state(config).

This also fixes a bug where a one-shot timer message whose delivery
failed once would get lost for good.

Signed-off-by: Andrey Smetanin <asmetanin@virtuozzo.com>
Reviewed-by: Roman Kagan <rkagan@virtuozzo.com>
CC: Gleb Natapov <gleb@kernel.org>
CC: Paolo Bonzini <pbonzini@redhat.com>
CC: Roman Kagan <rkagan@virtuozzo.com>
CC: Denis V. Lunev <den@openvz.org>
CC: qemu-devel@nongnu.org
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>