qemu-thread: Assert locks are initialized before using

Not all platforms check whether a lock is initialized before used.  In
particular Linux seems to be more permissive than OSX.

Check initialization state explicitly in our code to catch such bugs
earlier.

Signed-off-by: Fam Zheng <famz@redhat.com>
Message-Id: <20170704122325.25634-1-famz@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>