meson: use .allowed() method for features

The method is now in 0.59, using it simplifies some boolean conditions.
The other new methods .require() and .disable_auto_if() can be used too,
but introducing them is not just a matter of search-and-replace.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

tests/meson.build

@@ -85,7 +85,7 @@ if 'CONFIG_TCG' in config_all
   subdir('fp')
 endif
 
-if not get_option('tcg').disabled()
+if get_option('tcg').allowed()
   if 'CONFIG_PLUGIN' in config_host
     subdir('plugin')
   endif