Update qemu_cfg_read to use "rep insb".

Use rep insb instead of manual loop - the host may be able to optimize
the rep insb instruction.
diff --git a/src/paravirt.c b/src/paravirt.c
index da72c71..ca646d4 100644
--- a/src/paravirt.c
+++ b/src/paravirt.c
@@ -24,8 +24,7 @@
 static void
 qemu_cfg_read(u8 *buf, int len)
 {
-    while (len--)
-        *(buf++) = inb(PORT_QEMU_CFG_DATA);
+    insb(PORT_QEMU_CFG_DATA, buf, len);
 }
 
 static void