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

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

vcpu_show_registers() didn't do anything for HVM so far. Note though
that some extra hackery is needed for VMX - see the code comment.

Note further that the show_guest_stack() invocation is left alone here:
While strictly speaking guest_kernel_mode() should be predicated by a
PV / !HVM check, show_guest_stack() itself will bail immediately for
HVM.

While there and despite not being PVH-specific, take the opportunity and
filter offline vCPU-s: There's not really any register state associated
with them, so avoid spamming the log with useless information while
still leaving an indication of the fact.

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