is_selinux_enabled() returns -1 on error, account for this.

Per the documentation, is_selinux_enabled() returns -1 on error.
Account for this. Previously when -1 was being returned the condition
would still be true. I was noticing this because on my system that has
selinux disabled I was getting this in the libvirt.log every 5
seconds:

error : virIdentityGetSystem:173 : Unable to lookup SELinux process context: Invalid argument

With this patch applied, I no longer get these messages every 5
seconds. I am submitting this in case its deemed useful for inclusion.
Anyone have any comments on this change? This is a patch off current
master.
Signed-off-by: NMichal Privoznik <mprivozn@redhat.com>

src/security/security_selinux.c

@@ -784,7 +784,7 @@ error:
 static int
 virSecuritySELinuxSecurityDriverProbe(const char *virtDriver)
 {
-    if (!is_selinux_enabled())
+    if (is_selinux_enabled() <= 0)
         return SECURITY_DRIVER_DISABLE;
 
     if (virtDriver && STREQ(virtDriver, "LXC")) {

src/util/viridentity.c

@@ -169,7 +169,7 @@ virIdentityPtr virIdentityGetSystem(void)
         goto cleanup;
 
 #if WITH_SELINUX
-    if (is_selinux_enabled()) {
+    if (is_selinux_enabled() > 0) {
         if (getcon(&con) < 0) {
             virReportSystemError(errno, "%s",
                                  _("Unable to lookup SELinux process context"));