vmspawn: add --cxl= option and memory hotplug support

Add --cxl=BOOL option to enable CXL (Compute Express Link) support in
the virtual machine. CXL is a high-speed interconnect standard that
allows CPUs to access memory attached to devices such as accelerators
and memory expanders, enabling flexible memory pooling and expansion
beyond what is physically installed on the motherboard. When enabled,
adds cxl=on to the QEMU machine configuration. Only supported on x86_64
and aarch64 architectures.

This is added for testing purposes and for feature parity with mkosi's
CXL= setting.

Extend --ram= to accept an optional maximum size for memory hotplug,
using the syntax --ram=SIZE[:MAXSIZE] (e.g. --ram=2G:8G). When a
maximum is specified, the maxmem key is added to the QEMU memory
configuration section to enable memory hotplug up to the given limit.

Co-developed-by: Claude Opus 4.6 <noreply@anthropic.com>

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 --vsock --tpm --discard-disk --register --pass-ssh-key'
+        [BOOL]='--kvm --cxl --vsock --tpm --discard-disk --register --pass-ssh-key'
         [SECURE_BOOT]='--secure-boot'
         [FIRMWARE]='--firmware'
         [FIRMWARE_FEATURES]='--firmware-features'