KVM: selftests: Drop @slot0_mem_pages from __vm_create_with_vcpus()

All callers of __vm_create_with_vcpus() pass DEFAULT_GUEST_PHY_PAGES for
@slot_mem_pages; drop the param and just hardcode the "default" as the
base number of pages for slot0.

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