x86/PVH: actually show Dom0's stacks from debug key '0'

x86/PVH: actually show Dom0's stacks from debug key '0'

show_guest_stack() does nothing for HVM. Introduce a HVM-specific
dumping function, paralleling the 64- and 32-bit PV ones. We don't know
the real stack size, so only dump up to the next page boundary.

Rather than adding a vcpu parameter to hvm_copy_from_guest_linear(),
introduce hvm_copy_from_vcpu_linear() which - for now at least - in
return won't need a "pfinfo" parameter.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Roger Pau Monné <roger.pau@citrix.com>