Introduce some hooks for the shared part of qemu thread between POSIX
and Windows implementations.  Note that in qemu_mutex_unlock_impl() we
moved the call before unlock operation which should make more sense.
And we don't need qemu_mutex_post_unlock() hook.

Put all these shared hooks into the header files.  It should be internal
to qemu-thread but not for qemu-thread users, hence put into util/
directory.

Reviewed-by: Emilio G. Cota <cota@braap.org>
Signed-off-by: Peter Xu <peterx@redhat.com>
Message-Id: <20180425025459.5258-3-peterx@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>