ui/vdagent: add migration blocker when machine version < 10.1

In QEMU 10.1, commit 5d56bff11e ("ui/vdagent: add migration support")
added migration support for the vdagent chardev and commit 42000e0013
("ui/vdagent: remove migration blocker") removed the migration
blocker. No compat for older machine versions was added, so migration
with pre-10.1 machine version, from a 10.1 binary to a pre-10.1 binary
will result in a failure when loading the VM state in the target
instance:

> Unknown savevm section or instance 'vdagent' 0. Make sure that your
> current VM setup matches your saved VM setup, including any
> hotplugged devices

Add a compat flag to block migration when the machine version is less
than 10.1 to avoid this.

Cc: qemu-stable@nongnu.org
Fixes: 42000e0013 ("ui/vdagent: remove migration blocker")
Signed-off-by: Fiona Ebner <f.ebner@proxmox.com>
Reviewed-by: Fabiano Rosas <farosas@suse.de>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-Id: <20260310142552.240877-1-f.ebner@proxmox.com>