KVM: selftests: Randomly force emulation on x86 writes from guest code

Override vcpu_arch_put_guest() to randomly force emulation on supported
accesses.  Force emulation of LOCK CMPXCHG as well as a regular MOV to
stress KVM's emulation of atomic accesses, which has a unique path in
KVM's emulator.

Arbitrarily give all the decisions 50/50 odds; absent much, much more
sophisticated infrastructure for generating random numbers, it's highly
unlikely that doing more than a coin flip with affect selftests' ability
to find KVM bugs.

This is effectively a regression test for commit 910c57df

 ("KVM: x86:
Mark target gfn of emulated atomic instruction as dirty").

Link: https://lore.kernel.org/r/20240314185459.2439072-6-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>