Add USB emulation

[StrikerX3]

XQEMU's USB emulation and XID are necessary for input emulation.

Also check out QEMU's USB emulation.

Another possibly useful source is VirtualBox.

There was also ergo720's OHCI-LLE branch (user deleted) which has been merged with Cxbx-Reloaded's develop branch a while ago. It doesn't work yet, but may serve as a starting point.

Of course, it's also possible to go straight to the source and read the specifications: here or here.

[PatrickvL]

Recently, Cxbx-Reloaded was extended to go further with LLE USB - see Cxbx-Reloaded/Cxbx-Reloaded#1526

[StrikerX3]

I'm planning on rewriting the entire OHCI / XID emulation based on the official specs just like I did with IDE emulation, which works very well thanks to excellent documentation. XID is of course not well documented, so there will be a bit of research to get it to work.

[StrikerX3]

For XID, I'll use the XQEMU adapter (#22) to take advantage of their emulation.