x86/pv: Inject #GP for implicit grant unmaps

x86/pv: Inject #GP for implicit grant unmaps

This is a debug behaviour to identify buggy kernels.  Crashing the domain is
the most unhelpful thing to do, because it discards the relevant context.

Instead, inject #GP[0] like other permission errors in x86.  In particular,
this lets the kernel provide a backtrace which is more likely to be helpful to
a developer.

As a bugfix, this always injects #GP[0] to current, not l1e_owner.  It is not
l1e_owner's fault if dom0 using superpowers triggers an implicit unmap.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: Jan Beulich <jbeulich@suse.com>