Main/Serial+Devices/Serial: new TCP backend for serial port. Contributed by Alexey Eromenko. Thanks! Frontends/VirtualBox+VBoxManage: adapted accordingly

Main/MediumAttachment+Machine: stub attribute/method for an explicit hot-pluggable medium attachment flag, to be used by AHCI soon

Main/SystemProperties+Machine: new config setting for default VM frontend. Frontend/VirtualBox+VBoxManage: changes to use the default VM frontend when starting a VM, other minor cleanups Main/xml/*.xsd: attempt to bring the XML schema close to reality doc/manual: document the new possibilities, and fix a few long standing inaccuracies