configure: fix libpmem configuration option

For some reason, libpmem option setting was set to work in an opposite
way (--enable-libpmem disabled it and vice versa). Fixing this so
configuration works properly.

Signed-off-by: Miroslav Rezanina <mrezanin@redhat.com>
Reviewed-by: Connor Kuehl <ckuehl@redhat.com>
Reviewed-by: Pankaj Gupta <pankaj.gupta@ionos.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Tested-by: Connor Kuehl <ckuehl@redhat.com>
Message-Id: <20210707075144.82717-1-mrezanin@redhat.com>
Fixes: e36e8c7 ("configure, meson: convert libpmem detection to meson", 2021-07-06)
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

configure

@@ -1501,9 +1501,9 @@ for opt do
   ;;
   --disable-debug-mutex) debug_mutex=no
   ;;
-  --enable-libpmem) libpmem=disabled
+  --enable-libpmem) libpmem="enabled"
   ;;
-  --disable-libpmem) libpmem=enabled
+  --disable-libpmem) libpmem="disabled"
   ;;
   --enable-xkbcommon) xkbcommon="enabled"
   ;;