error: Use error_report_err() where appropriate

Coccinelle semantic patch:

    @@
    expression E;
    @@
    -    error_report("%s", error_get_pretty(E));
    -    error_free(E);
    +    error_report_err(E);
    @@
    expression E, S;
    @@
    -    error_report("%s", error_get_pretty(E));
    +    error_report_err(E);
    (
         exit(S);
    |
         abort();
    )

Trivial manual touch-ups in block/sheepdog.c.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
diff --git a/qemu-char.c b/qemu-char.c
index 98d4342..f901ed1 100644
--- a/qemu-char.c
+++ b/qemu-char.c
@@ -3753,8 +3753,7 @@
 
     chr = qemu_chr_new_from_opts(opts, init, &err);
     if (err) {
-        error_report("%s", error_get_pretty(err));
-        error_free(err);
+        error_report_err(err);
     }
     if (chr && qemu_opt_get_bool(opts, "mux", 0)) {
         qemu_chr_fe_claim_no_fail(chr);