xend: fix avoidance to restart domain on crash

xend: fix avoidance to restart domain on crash

If a qemu-dm dies immediately (probably by wrong setting),
xend repeats to restart a domain so many times.=20
That causes system overload.

There is already a logic to avoid too early restarting, however,
it might not work. Since xenstore entry 'xend/previous_restart_time'
is volatile. XendDomainInfo.destroy() which removes the entry from
xenstore is called in some places.

Also, this patch prevents too early restarting even at the first
domain creation.

Signed-off-by: Kouya Shimura <kouya@jp.fujitsu.com>