meson: create have_vhost_* variables

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>

tests/meson.build

@@ -68,7 +68,7 @@ test_deps = {
   'test-qht-par': qht_bench,
 }
 
-if have_tools and 'CONFIG_VHOST_USER' in config_host and 'CONFIG_LINUX' in config_host
+if have_tools and have_vhost_user and 'CONFIG_LINUX' in config_host
   executable('vhost-user-bridge',
              sources: files('vhost-user-bridge.c'),
              dependencies: [qemuutil, vhost_user])