vmware: add legacy text-clipboard backdoor commands (6-9)

[felixrieseberg]

Just like the other VMWare-related PRs, no hard feelings if you don't want them - I just want to upstream stuff I've done for Windows. This one implements the four legacy text-clipboard backdoor commands on VMware port 0x5658:

Cmd	Name	Direction	Description
6	GETSELLENGTH	host→guest	Returns byte-length of staged text, 0xFFFFFFFF if nothing fresh
7	GETNEXTPIECE	host→guest	Reads next 4 bytes of staged text (LE dword)
8	SETSELLENGTH	guest→host	Allocates a receive buffer for an incoming transfer
9	SETNEXTPIECE	guest→host	Writes next 4 bytes into the receive buffer; emits vmware-clipboard-guest when full

Bus events added:

• vmware-clipboard-host – caller stages a Uint8Array; the device makes it readable via GETSELLENGTH/GETNEXTPIECE
• vmware-clipboard-guest – fired when the guest finishes writing, carries the received Uint8Array

This is the pre-RPC clipboard protocol that open-vm-tools and its predecessors use. Because the guest reads and writes directly via IN/OUT instructions on port 0x5658, a minimal Win9x agent can sync CF_TEXT with the host from ring 3 without any kernel driver — just a MOV EAX, 564D5868h / IN EAX, DX loop.

The command numbers and wire format match VMware Workstation's implementation as documented in the open-vm-tools source (lib/backdoor/backdoorGcc32.c, vmtools/src/vmbackdoor.c).

Implementation notes

• CLIP_MAX (64 KiB) caps both directions to prevent unbounded allocation.
• clip_out_fresh is a one-shot flag: GETSELLENGTH arms it (sets fresh = false) so a second poll before new data arrives returns 0xFFFFFFFF.
• On the guest→host side, SETNEXTPIECE auto-fires vmware-clipboard-guest once the cursor reaches the declared length. A zero-length SETSELLENGTH fires immediately.
• Clipboard state is ephemeral and not included in get_state/set_state; a save/restore discards any in-flight transfer, matching the behaviour of real VMware Tools.

[chschnell]

Nice PR (same goes for #1542)!

Do you have some experience with vmware backdoor support for older guests? I was looking for Windows and found these:

• https://archive.org/details/winPreVista
• https://archive.org/details/VMwareToolsforWin95RTM_OSR1

I have the absolute mouse positioning working under Windows 3.1, I wonder if I could get the clipboard working, too.