Changes

Vagrant: Support host IO cache on SATA, and, optionaly, SSD-aware
features
Host IO cache: trade IO speed for security on e.g. host power failure, 
more here: https://www.virtualbox.org/manual/ch05.html#iocaching.
`--nonrotational` (optional): This switch allows to enable the 
non-rotational flag for virtual hard disks. Some guests (i.e. Windows
7+) treat such disks like SSDs and don't perform disk fragmentation on
such media.
`--discard` (not implemented): This switch enables the auto-discard 
feature for the virtual hard disks. This specifies that a VDI image will
be shrunk in response to the trim command from the guest OS.
https://www.virtualbox.org/manual/ch08.html#vboxmanage-storageattach
 (commit: f6ec4f3)