stubs: group all monitor_fdset_* functions in a single file

It makes little sense to implement only one of them, so avoid
proliferation of stubs files.

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

stubs/fdset.c

@@ -0,0 +1,22 @@
+#include "qemu/osdep.h"
+#include "qemu-common.h"
+#include "monitor/monitor.h"
+
+int monitor_fdset_dup_fd_add(int64_t fdset_id, int dup_fd)
+{
+    return -1;
+}
+
+int monitor_fdset_dup_fd_find(int dup_fd)
+{
+    return -1;
+}
+
+int monitor_fdset_get_fd(int64_t fdset_id, int flags)
+{
+    return -1;
+}
+
+void monitor_fdset_dup_fd_remove(int dupfd)
+{
+}