x86/HVM: properly deal with hvm_copy_*_guest_phys() errors

x86/HVM: properly deal with hvm_copy_*_guest_phys() errors

In memory read/write handling the default case should tell the caller
that the operation cannot be handled rather than the operation having
succeeded, so that when new HVMCOPY_* states get added not handling
them explicitly will not result in errors being ignored.

In task switch emulation code stop handling some errors, but not
others.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: Keir Fraser <keir@xen.org>