qga: conditionalize schema for commands requiring linux/win32

qga: conditionalize schema for commands requiring linux/win32

Some commands were blocked based on CONFIG_FSFREEZE, but their
impl had nothing todo with CONFIG_FSFREEZE, and were instead
either Linux-only, or Win+Linux-only.

Rather than creating stubs for every command that just return
QERR_UNSUPPORTED, use 'if' conditions in the QAPI schema to
fully exclude generation of the stats and fsinfo commands on
platforms that can't support them.

The command will be rejected at QMP dispatch time instead,
avoiding reimplementing rejection by blocking the stub commands.
This changes the error message for affected commands from

    {"class": "CommandNotFound", "desc": "Command FOO has been disabled"}

to

    {"class": "CommandNotFound", "desc": "The command FOO has not been found"}

This has the additional benefit that the QGA protocol reference
now documents what conditions enable use of the command.

Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>
Reviewed-by: Konstantin Kostiuk <kkostiuk@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Manos Pitsidianakis <manos.pitsidianakis@linaro.org>
Message-ID: <20240712132459.3974109-12-berrange@redhat.com>
Signed-off-by: Konstantin Kostiuk <kkostiuk@redhat.com>