KVM: selftests: Add x86-64 support for exception fixup

Add x86-64 support for exception fixup on single instructions, without
forcing tests to install their own fault handlers.  Use registers r9-r11
to flag the instruction as "safe" and pass fixup/vector information,
i.e. introduce yet another flavor of fixup (versus the kernel's in-memory
tables and KUT's per-CPU area) to take advantage of KVM sefltests being
64-bit only.

Using only registers avoids the need to allocate fixup tables, ensure
FS or GS base is valid for the guest, ensure memory is mapped into the
guest, etc..., and also reduces the potential for recursive faults due to
accessing memory.

Providing exception fixup trivializes tests that just want to verify that
an instruction faults, e.g. no need to track start/end using global
labels, no need to install a dedicated handler, etc...

Deliberately do not support #DE in exception fixup so that the fixup glue
doesn't need to account for a fault with vector == 0, i.e. the vector can
also indicate that a fault occurred.  KVM injects #DE only for esoteric
emulation scenarios, i.e. there's very, very little value in testing #DE.
Force any test that wants to generate #DEs to install its own handler(s).

Use kvm_pv_test as a guinea pig for the new fixup, as it has a very
straightforward use case of wanting to verify that RDMSR and WRMSR fault.

Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20220608224516.3788274-3-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>