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

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: Michal Privoznik <mprivozn@redhat.com>