KVM: selftests: Push vm_adjust_num_guest_pages() into "w/o vCPUs" helper

Move the call to vm_adjust_num_guest_pages() from vm_create_with_vcpus()
down into vm_create_without_vcpus().  This will allow a future patch to
make the "w/o vCPUs" variant the common inner helper, e.g. so that the
"with_vcpus" helper calls the "without_vcpus" helper, instead of having
them be separate paths.

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