fix "Missing break in switch" coverity reports

Many of these are marked as "intentional/fix required" because they
just need adding a fall through comment.  This is exactly what this
patch does, except for target/mips/translate.c where it is easier to
duplicate the code, and hw/audio/sb16.c where I consulted the DOSBox
sources and decide to just remove the LOG_UNIMP before the fallthrough.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

hw/arm/pxa2xx.c

@@ -409,7 +409,7 @@ static uint64_t pxa2xx_mm_read(void *opaque, hwaddr addr,
     case MDCNFG ... SA1110:
         if ((addr & 3) == 0)
             return s->mm_regs[addr >> 2];
-
+        /* fall through */
     default:
         printf("%s: Bad register " REG_FMT "\n", __func__, addr);
         break;