vdagent: clipboard: GlobalUnlock on handle_request failure

Moved clipboard definition to the beginning of the function, as
mingw-gcc complains about gotos that jump over definitions.
vdagent: clipboard: Add VD_AGENT_CAP_MAX_CLIPBOARD support

Do not send clipboard data bigger than last received
VDAgentMaxClipboard.

If a larger amount of data needs to be transferred between client and guest,
a different mechanism should be used, not clipboard.

The spice-protocol git-submodule is updated to include VD_AGENT_MAX_CLIPBOARD
and related definitions.
Merge branch 'master' of https://github.com/SPICE/win32-vd_agent