include: define constant for early constructor priority

Functions marked with __attribute__((__constructor__)) will be
invoked in linker order. In theory this is well defined, but
in practice, it is hard to determine what this order will be
with the layers of indirection through meson, ninja and the
static libraries QEMU builds.

Notably, the order currently appears different between Linux
and Windows (as tested with Wine on Linux). This can cause
problems when certain QEMU constructors have a dependancy on
other QEMU constructors.

To address this define a QEMU_CONSTRUCTOR_EARLY constant which
provides a priority value that will run before other default
constructors. This is to be used for QEMU constructors that
are themselves self-contained, but may be relied upon by other
constructors.

Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Dr. David Alan Gilbert <dave@treblig.org>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>

include/qemu/compiler.h

@@ -194,6 +194,14 @@
 # define QEMU_USED
 #endif
 
+/*
+ * A priority for __attribute__((constructor(...))) that
+ * will run earlier than the default constructors. Must
+ * only be used for functions that have no dependency
+ * on global initialization of other QEMU subsystems.
+ */
+#define QEMU_CONSTRUCTOR_EARLY 101
+
 /*
  * Disable -ftrivial-auto-var-init on a local variable.
  *