Add an optional "tls='yes|no'" attribute for a TCP chardev.

For QEMU, this will allow for disabling the host config setting of the
'chardev_tls' for a domain chardev channel by setting the value to "no" or
to attempt to use a host TLS environment when setting the value to "yes"
when the host config 'chardev_tls' setting is disabled, but a TLS environment
is configured via either the host config 'chardev_tls_x509_cert_dir' or
'default_tls_x509_cert_dir'
Signed-off-by: NJohn Ferlan <jferlan@redhat.com>
Signed-off-by: NPavel Hrdina <phrdina@redhat.com>