vl: change runstate only if new state is different from current state

Previously, qemu will abort at following scenario:
(qemu) stop
(qemu) system_reset
(qemu) system_reset
(qemu) 2016-04-13T20:54:38.979158Z qemu-system-x86_64: invalid runstate transition: 'prelaunch' -> 'prelaunch'

Signed-off-by: Li Zhijian <lizhijian@cn.fujitsu.com>
Acked-by: Paolo Bonzini <pbonzini@redhat.com>
Message-Id: <1460604352-18630-1-git-send-email-lizhijian@cn.fujitsu.com>
Cc: qemu-stable@nongnu.org
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>