KVM: selftests: Add option to overlap vCPU memory access

Add an option to overlap the ranges of memory each vCPU accesses instead
of partitioning them. This option will increase the probability of
multiple vCPUs faulting on the same page at the same time, and causing
interesting races, if there are bugs in the page fault handler or
elsewhere in the kernel.

Reviewed-by: Jacob Xu <jacobhxu@google.com>
Reviewed-by: Makarand Sonare <makarandsonare@google.com>

Signed-off-by: Ben Gardon <bgardon@google.com>
Message-Id: <20210112214253.463999-6-bgardon@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>