When using Meson options rather than config-host.h, the "when" clauses
have to be changed to if statements (which is not necessarily great,
though at least it highlights which parts of the build are per-target
and which are not).

Do that before moving vhost logic to meson.build, though for now
the variables are just based on config-host.mak data.

Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>