qemu: Check if AMD secure guest support is enabled

Implement secure guest check for AMD SEV (Secure Encrypted
Virtualization) in order to invalidate the qemu capabilities
cache in case the availability of the feature changed.

For AMD SEV the verification consists of:
 - checking if /sys/module/kvm_amd/parameters/sev contains the
   value '1': meaning SEV is enabled in the host kernel;
 - checking if /dev/sev exists

Signed-off-by: Paulo de Rezende Pinatti <ppinatti@linux.ibm.com>
Signed-off-by: Boris Fiuczynski <fiuczy@linux.ibm.com>
Reviewed-by: Bjoern Walk <bwalk@linux.ibm.com>
Reviewed-by: Erik Skultety <eskultet@redhat.com>

src/qemu/qemu_capabilities.c

@@ -4715,6 +4715,27 @@ virQEMUCapsKVMSupportsSecureGuestS390(void)
 }
 
 
+/*
+ * Check whether AMD Secure Encrypted Virtualization (x86) is enabled
+ */
+static bool
+virQEMUCapsKVMSupportsSecureGuestAMD(void)
+{
+    g_autofree char *modValue = NULL;
+
+    if (virFileReadValueString(&modValue, "/sys/module/kvm_amd/parameters/sev") < 0)
+        return false;
+
+    if (modValue[0] != '1')
+        return false;
+
+    if (virFileExists(QEMU_DEV_SEV))
+        return true;
+
+    return false;
+}
+
+
 /*
  * Check whether the secure guest functionality is enabled.
  * See the specific architecture function for details on the verifications made.
@@ -4726,6 +4747,10 @@ virQEMUCapsKVMSupportsSecureGuest(void)
 
     if (ARCH_IS_S390(arch))
         return virQEMUCapsKVMSupportsSecureGuestS390();
+
+    if (ARCH_IS_X86(arch))
+        return virQEMUCapsKVMSupportsSecureGuestAMD();
+
     return false;
 }