qemu-char: Fix ringbuf option size

Any attempt to use it trips an "opt->desc->type == QEMU_OPT_NUMBER"
assertion.  Broken in commit 1da48c65.

Cc: qemu-stable@nongnu.org
Signed-off-by: NMarkus Armbruster <armbru@redhat.com>
Reviewed-by: NEric Blake <eblake@redhat.com>
Signed-off-by: NLuiz Capitulino <lcapitulino@redhat.com>

qemu-char.c

@@ -3115,7 +3115,7 @@ static void qemu_chr_parse_memory(QemuOpts *opts, ChardevBackend *backend,
 
     backend->memory = g_new0(ChardevMemory, 1);
 
-    val = qemu_opt_get_number(opts, "size", 0);
+    val = qemu_opt_get_size(opts, "size", 0);
     if (val != 0) {
         backend->memory->has_size = true;
         backend->memory->size = val;