qemu-char: introduce qemu_chr_alloc

The next patch will modify this function to initialize state that is
common to all backends.

Reviewed-by: Fam Zheng <famz@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>
diff --git a/spice-qemu-char.c b/spice-qemu-char.c
index 6624559..4518a4d 100644
--- a/spice-qemu-char.c
+++ b/spice-qemu-char.c
@@ -268,7 +268,7 @@
     CharDriverState *chr;
     SpiceCharDriver *s;
 
-    chr = g_malloc0(sizeof(CharDriverState));
+    chr = qemu_chr_alloc();
     s = g_malloc0(sizeof(SpiceCharDriver));
     s->chr = chr;
     s->active = false;