KVM: selftests: Support multiple vCPUs in demand paging test

Most VMs have multiple vCPUs, the concurrent execution of which has a
substantial impact on demand paging performance. Add an option to create
multiple vCPUs to each access disjoint regions of memory.
Signed-off-by: Ben Gardon <bgardon@google.com>
[guest_code() can't return, use GUEST_ASSERT(). Ensure the number
 of guests pages is compatible with the host.]
Signed-off-by: Andrew Jones <drjones@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>