Since TLS was introduced hostwide for libvirt 2.3.0 and a domain
configurable haveTLS was implemented for libvirt 2.4.0, we have to
modify the migratable XML for specific case where the 'tls' attribute
is based on setting from qemu.conf.

The "tlsFromConfig" is libvirt internal attribute and is stored only in
status XML to ensure that when libvirtd is restarted this internal flag
is not lost by the restart.

That flag is used to decide whether we should put *tls* attribute to
migratable XML or not.
Signed-off-by: NPavel Hrdina <phrdina@redhat.com>