KVM: selftests: Create VM with adjusted number of guest pages for perf tests

Use the already computed guest_num_pages when creating the so called
extra VM pages for a perf test, and add a comment explaining why the
pages are allocated as extra pages.

Signed-off-by: Sean Christopherson <seanjc@google.com>
Reviewed-by: Ben Gardon <bgardon@google.com>
Signed-off-by: David Matlack <dmatlack@google.com>
Message-Id: <20211111000310.1435032-11-dmatlack@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>