x86: improve output resulting from sending '0' over serial

x86: improve output resulting from sending '0' over serial

While the original logic already implied that the kernel part of the
guest's address space is identical on all vCPU-s (i.e. for all guest
processes), it didn't fully leverage the potential here: As long as
the top page table currently active is owned by the subject domain
(currently only Dom0), the stack dump can be done without extra
effort.

For x86-64, additionally add page table traversal so that the stack
can be dumped in all cases (unless it's invalid or user space).

I left the 32-bit variant of do_page_walk() unimplemented for the
moment as I couldn't convince myself using map_domain_page() there is
a good idea, and didn't want to introduce new fixmap entries either.

Signed-off-by: Jan Beulich <jbeulich@novell.com>