KVM: selftests: Add support for #DE exception fixup

KVM: selftests: Add support for #DE exception fixup

Add support for handling #DE (divide error) exceptions in KVM selftests
so that the fastops test can verify KVM correctly handles #DE when
emulating DIV or IDIV on behalf of the guest.  Morph #DE to 0xff (i.e.
to -1) as a mostly-arbitrary vector to indicate #DE, so that '0' (the
real #DE vector) can still be used to indicate "no exception".

Link: https://lore.kernel.org/r/20250909202835.333554-2-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>