DM: fix build issue with gcc 4.8.4 on ubuntu 14.04

There are two build issues:
- add -fno-strict-aliasing to address
  error: dereferencing type-punned pointer will break strict-aliasing rules

- initialize tfd to zero to address
  error: ‘tfd’ may be used uninitialized in this function

Signed-off-by: Yin Fengwei <fengwei.yin@intel.com>

devicemodel/hw/pci/ahci.c

@@ -1478,7 +1478,7 @@ static void
 atapi_mode_sense(struct ahci_port *p, int slot, uint8_t *cfis)
 {
 	uint8_t *acmd;
-	uint32_t tfd;
+	uint32_t tfd = 0;
 	uint8_t pc, code;
 	int len;