check for cfg->spiceTLS earlier in qemuProcessSPICEAllocatePorts

check for cfg->spiceTLS earlier in qemuProcessSPICEAllocatePorts

This saves a few lines of code and catches the error when:
<spice autoport ='yes' defaultMode='any' ..>
  <channel name='main' mode='secure'/>
</spice>
is specified with spice_tls = 0 in qemu.conf.

Instead of this error in qemuBuildGraphicsSPICECommandLine:
error: unsupported configuration: spice secure channels set in XML
configuration, but TLS port is not provided

an error is reported in qemuProcessSPICEAllocatePorts:
error: unsupported configuration: Auto allocation of spice TLS port
requested but spice TLS is disabled in qemu.conf

Inspired by:
https://www.redhat.com/archives/libvir-list/2014-June/msg01408.html