KVM: selftests: Drop DEFAULT_GUEST_PHY_PAGES, open code the magic number

Remove DEFAULT_GUEST_PHY_PAGES and open code the magic number (with a
comment) in vm_nr_pages_required().  Exposing DEFAULT_GUEST_PHY_PAGES to
tests was a symptom of the VM creation APIs not cleanly supporting tests
that create runnable vCPUs, but can't do so immediately.  Now that tests
don't have to manually compute the amount of memory needed for basic
operation, make it harder for tests to do things that should be handled
by the framework, i.e. force developers to improve the framework instead
of hacking around flaws in individual tests.

Signed-off-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>