xend: fix "xm list hangs"

xend: fix "xm list hangs"

If a command hold domains_lock, "xm list" would hang for waiting for
the lock. Such as creating many VMs at a script (such as 20), command
of "xm list" could hang for long time(10 mins).  I think domains_lock
here only protect update().  So, we shouldn't do update before command
of "list" really get this lock, but xm do need show the domain's
information quickly. In this patch, if command couldn't get the
domains_lock after 20 times trying, "xm list" would show the
information of VMs without update().

Signed-off-by: James Song (Wei) <jsong@novell.com>