util: ensure virMutexInit is not recursive

util: ensure virMutexInit is not recursive

POSIX states that creation of a mutex with default attributes
is unspecified whether the mutex is recursive or non-recursive.
We specifically want non-recursive (deadlock is desirable in
flushing out coding bugs that used our mutex incorrectly).

* src/util/threads-pthread.c (virMutexInit): Specifically request
non-recursive mutex, rather than relying on unspecified default.