qemu-progress.c: printf isn't signal safe

Change the signal handling to indicate a signal is pending, rather
then printing directly from the signal handler.

In addition make the signal prints go to stderr, rather than stdout.

Signed-off-by: Jes Sorensen <Jes.Sorensen@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

qemu-progress.c

diff --git a/qemu-progress.c b/qemu-progress.c
index e1feb89..a4894c0 100644
--- a/qemu-progress.c
+++ b/qemu-progress.c
@@ -37,6 +37,7 @@
 };
 
 static struct progress_state state;
+static volatile sig_atomic_t print_pending;
 
 /*
  * Simple progress print function.
@@ -63,12 +64,16 @@
 #ifdef CONFIG_POSIX
 static void sigusr_print(int signal)
 {
-    printf("    (%3.2f/100%%)\n", state.current);
+    print_pending = 1;
 }
 #endif
 
 static void progress_dummy_print(void)
 {
+    if (print_pending) {
+        fprintf(stderr, "    (%3.2f/100%%)\n", state.current);
+        print_pending = 0;
+    }
 }
 
 static void progress_dummy_end(void)