x86/PV: replace assertions in '0' debug key stack dumping

x86/PV: replace assertions in '0' debug key stack dumping

While it was me to add them, I'm afraid I don't see justification for
the assertions: A vCPU may very well have got preempted while in user
mode. Limit compat guest user mode stack dumps to the containing page
(like is done when using do_page_walk()), and suppress user mode stack
dumping altogether for 64-bit domains.

Fixes: cc0de53a903c ("x86: improve output resulting from sending '0' over serial")
Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Roger Pau Monné <roger.pau@citrix.com>