Add Guest Additions support

[Zedeldi]

See IGuest, accessible from a machine's IConsole object.
Allow authenticating with guest OS, file system I/O, process information, etc.

[Zedeldi]

Reading guest property values support added in b92d0f4 and 01ceba3.
Leaving issue open to track progress of interaction with guest OS, e.g. authentication, file system I/O, process information, etc.
Also add shortcut to insert guest additions CD image.

[Zedeldi]

Support to attach/detach guest additions CD image added via 91536c8.