vmspawn: Always enable CXL on supported architectures

Drop the --cxl= option and unconditionally enable cxl=on the QEMU
machine type whenever the host architecture supports it (x86_64 and
aarch64). The flag was only added for testing parity with mkosi's CXL=
setting and there is no reason to leave it as an opt-in toggle: with no
pxb-cxl device or cxl-fmw window attached, enabling it on the machine
only reserves a small MMIO region and emits an empty CEDT, so the cost
is negligible while removing one knob users would otherwise have to
flip explicitly to exercise the CXL code paths in QEMU.

shell-completion/bash/systemd-vmspawn

@@ -31,7 +31,7 @@ _systemd_vmspawn() {
     local -A OPTS=(
         [STANDALONE]='-h --help --version -q --quiet --no-pager -n --network-tap --network-user-mode --user --system -x --ephemeral'
         [PATH]='-D --directory -i --image --linux --initrd --extra-drive --forward-journal --efi-nvram-template'
-        [BOOL]='--kvm --cxl --vsock --tpm --discard-disk --pass-ssh-key'
+        [BOOL]='--kvm --vsock --tpm --discard-disk --pass-ssh-key'
         [TRISTATE]='--register --secure-boot'
         [FIRMWARE]='--firmware'
         [FIRMWARE_FEATURES]='--firmware-features'