Documentation: update config name in real-time architecture support

Documentation: update config name in real-time architecture support

Commit 4843a45ef9fe8 ("entry: Rename "kvm" entry code assets to "virt"
to genericize APIs") renames the config KVM_XFER_TO_GUEST_WORK to
VIRT_XFER_TO_GUEST_WORK.

Adjust the documentation to the current situation, and specifically
refer to the new name of the config.

Signed-off-by: Lukas Bulwahn <lukas.bulwahn@redhat.com>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Message-ID: <20251127154343.292156-3-bigeasy@linutronix.de>