util: use fcntl() for qemu_write_pidfile() locking

Daniel Berrangé suggested to use fcntl() locks rather than lockf().

'man lockf':

   On Linux, lockf() is just an interface on top of fcntl(2) locking.
   Many other systems implement lockf() in this way, but note that
   POSIX.1 leaves the relationship between lockf() and fcntl(2) locks
   unspecified.  A portable application should probably avoid mixing
   calls to these interfaces.

IOW, if its just a shim around fcntl() on many systems, it is clearer
if we just use fcntl() directly, as we then know how fcntl() locks will
behave if they're on a network filesystem like NFS.

Suggested-by: Daniel P. Berrangé <berrange@redhat.com>
Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-Id: <20180831145314.14736-3-marcandre.lureau@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

util/oslib-posix.c

@@ -95,6 +95,11 @@ bool qemu_write_pidfile(const char *path, Error **errp)
 
     while (1) {
         struct stat a, b;
+        struct flock lock = {
+            .l_type = F_WRLCK,
+            .l_whence = SEEK_SET,
+            .l_len = 0,
+        };
 
         fd = qemu_open(path, O_CREAT | O_WRONLY, S_IRUSR | S_IWUSR);
         if (fd == -1) {
@@ -107,7 +112,7 @@ bool qemu_write_pidfile(const char *path, Error **errp)
             goto fail_close;
         }
 
-        if (lockf(fd, F_TLOCK, 0) < 0) {
+        if (fcntl(fd, F_SETLK, &lock)) {
             error_setg_errno(errp, errno, "Cannot lock pid file");
             goto fail_close;
         }