No sense outputting the qemu-ga and qemu-ga-ref man pages when the guest
agent binary itself is disabled. This mirrors behaviour from before the
meson switch.

Signed-off-by: Stefan Reiter <s.reiter@proxmox.com>
Message-Id: <20210128145801.14384-1-s.reiter@proxmox.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>