handle memory allocation failure in ktapvm

Normally we don't want to trace under memory pressure,
so we use a simple rule to handle memory allocation failure:

retry until allocation success, this will make caller don't need
to handle the unlikely failure case, then ktap exit.

In this approach, if user find there have memory allocation failure,
user should re-run the ktap script, or fix the memory pressure
issue, or figure out why the script need so many memory.

Perhaps return pre-allocated stub memory trunk when allocate failed
is a better approch in future?

Signed-off-by: zhangwei(Jovi) <jovi.zhangwei@gmail.com>