virtcontainers: Add support for Secure Execution

Secure Execution is a confidential computing technology on s390x (IBM Z
& LinuxONE). Enable the correspondent virtualization technology in QEMU
(where it is referred to as "Protected Virtualization").

- Introduce enableProtection and appendProtectionDevice functions for
  QEMU s390x.
- Introduce CheckCmdline to check for "prot_virt=1" being present on the
  kernel command line.
- Introduce CPUFacilities and avilableGuestProtection for hypervisor
  s390x to check for CPU support.

Fixes: #1771

Signed-off-by: Jakob Naucke <jakob.naucke@ibm.com>

src/runtime/virtcontainers/qemu_amd64_test.go

@@ -299,6 +299,12 @@ func TestQemuAmd64AppendProtectionDevice(t *testing.T) {
 	assert.Error(err)
 	assert.Empty(bios)
 
+	// Secure Execution protection
+	amd64.(*qemuAmd64).protection = seProtection
+	devices, bios, err = amd64.appendProtectionDevice(devices, firmware)
+	assert.Error(err)
+	assert.Empty(bios)
+
 	// sev protection
 	// TODO: update once it's supported
 	amd64.(*qemuAmd64).protection = sevProtection