testing, gdbstub and plugin updates - enable more sbsa-ref tests in avocado - add swtpm to the package lists - reduce avocado noise in gitlab by limiting tests - make docker engine choice driven by configure and enable override - remove unneeded gcc suffix on some cross compilers - fix some NULL returns in gdbstub - improve locking in execlog plugin - introduce the GDBFeature structure - consistently set gdb_core_xml_file - use cleaner escaping for gdb xml - drop ancient gdb_has_xml() test - disable multi-instruction GUSA emulation when plugins enabled - fix some coverity issues in plugins