KVM: selftests: Introduce a VM_MODE_DEFAULT macro for the default bits

This will be required later for tests like the kvm_create_max_vcpus
test that do not use the vm_create_default() function.

Reviewed-by: Andrew Jones <drjones@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>
Message-Id: <20190523164309.13345-5-thuth@redhat.com>
Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>