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>