qemu-io: Use getopt() correctly

POSIX says getopt() returns -1 on completion.  While Linux happens
to define EOF as -1, this definition is not required by POSIX, and
there may be platforms where checking for EOF instead of -1 would
lead to an infinite loop.

Signed-off-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Alberto Garcia <berto@igalia.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
diff --git a/qemu-io.c b/qemu-io.c
index 8e41080..ae5e274 100644
--- a/qemu-io.c
+++ b/qemu-io.c
@@ -120,7 +120,7 @@
     QemuOpts *qopts;
     QDict *opts;
 
-    while ((c = getopt(argc, argv, "snrgo:")) != EOF) {
+    while ((c = getopt(argc, argv, "snrgo:")) != -1) {
         switch (c) {
         case 's':
             flags |= BDRV_O_SNAPSHOT;