devel/libvirt: unbreak with --auto-features=enabled on i386

meson.build:1438:2: ERROR: Dependency "xenlight" not found, tried pkgconfig

As XEN option is limited to amd64 the option helper doesn't trigger,
so pass the argument more than once on amd64.

$ make -V '${MESON_ARGS:M*libxl*}' WITH=XEN
-Ddriver_libxl=disabled -Ddriver_libxl=enabled

PR: 270361
Reported by: antoine (via exp-run)