qemu: fix domxml-to-native failing when spice_tls is not enabled

qemu: fix domxml-to-native failing when spice_tls is not enabled

The default graphics channel mode is 'any', so as to defaultMode attribute.
If defaultMode and channel mode are all the default value 'any',
qemuConnectDomainXMLToNative will set TLSPort.
But in qemuBuildGraphicsSPICECommandLine, if spice_tls is not enabled, libvirtd
will report an error to tell the user that spice TLS is disabled in qemu.conf.

So qemuConnectDomainXMLToNative should check spice_tls is enabled,
then decide to allocate an tlsPort number to this graphics.

If user specified defaultMode is 'secure', qemuConnectDomainXMLToNative
could allocate tlsPort, and then let qemuBuildGraphicsSPICECommandLine reports
the spice_tls disabled error.

The related bug is:
https://bugzilla.redhat.com/show_bug.cgi?id=1113868

Signed-off-by: Jincheng Miao <jmiao@redhat.com>
Signed-off-by: Ján Tomko <jtomko@redhat.com>