KVM: selftests: Factor out guest mode code

demand_paging_test, dirty_log_test, and dirty_log_perf_test have
redundant guest mode code. Factor it out.

Also, while adding a new include, remove the ones we don't need.

Reviewed-by: Ben Gardon <bgardon@google.com>
Reviewed-by: Peter Xu <peterx@redhat.com>
Signed-off-by: Andrew Jones <drjones@redhat.com>
Message-Id: <20201218141734.54359-2-drjones@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>