xenoprof: Handle 32-bit guest stacks properly in a 64-bit hypervisor

xenoprof: Handle 32-bit guest stacks properly in a 64-bit hypervisor

The dump_guest_backtrace() function attempted to walk the stack
based on the assumption that the guest and hypervisor pointer sizes
were the same; thus any 32-bit guest running under 64-bit hypervisor
would have unreliable results.

In 64-bit mode, read the 32-bit stack frame properly.

Signed-off-by: Marcus Granado <marcus.granado@eu.citrix.com>
Signed-off-by: George Dunlap <george.dunlap@eu.citrix.com>
Committed-by: Keir Fraser <keir@xen.org>