x86/PV: make '0' debug key dump Dom0's stacks again

x86/PV: make '0' debug key dump Dom0's stacks again

The conversion to __get_guest() failed to account for the fact that for
remote vCPU-s dumping gets done through a pointer obtained from
map_domain_page(): __get_guest() arranges for (apparent) accesses to
hypervisor space to cause #GP(0).

Fixes: 6a1d72d3739e ('x86: split __{get,put}_user() into "guest" and "unsafe" variants')
Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Roger Pau Monné <roger.pau@citrix.com>