Fix build with newer version of GNUTLS

Fix build with newer version of GNUTLS

gnutls_kx_set_priority, gnutls_certificate_type_set_priority and
gnutls_protocol_set_priority were deprecated and eventually removed in
GNUTLS 3.4. Application should use gnutls_priority_set_direct instead
per [0].

gnutls_anon_server_credentials was deprecated at some point. Application
should use gnutls_anon_server_credentials_t instead.

Provide compatibility layer for QEMU traditional. This commit is in fact
backport of two upstream QEMU commits:
1. f40d55081667a716312b9a8b6e13835c4074f56b
2. 7d2a929feba319c18603e324b1750830d6c8b7a1

[0] https://www.gnutls.org/manual/html_node/Upgrading-from-previous-versions.html

Signed-off-by: Sjoer van der Ploeg <sfjuocekr@gmail.com>
Signed-off-by: Wei Liu <wei.liu2@citrix.com>
Tested-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>