When spice_tls is set but listen_tls is not, we don't initialize
GnuTLS library. So any later gnutls call (e.g. during migration,
where we initialize a certificate) will access uninitialized GnuTLS
internal structs and throws an error.

Although, we might now initialize GnuTLS twice, it is safe according
to the documentation:

    This function can be called many times,
    but will only do something the first time.

This patch creates 2 functions: virNetTLSInit and virNetTLSDeinit
with respect to written above.