Fix some typos in comments and documentation

helpfull -> helpful
usefull -> useful
cotrol -> control

and a grammar fix.
Signed-off-by: NStefan Weil <weil@mail.berlios.de>
Signed-off-by: NStefan Hajnoczi <stefanha@linux.vnet.ibm.com>

qemu-options.hx

@@ -937,8 +937,8 @@ a lot of bandwidth at the expense of quality.
 Disable adaptive encodings. Adaptive encodings are enabled by default.
 An adaptive encoding will try to detect frequently updated screen regions,
 and send updates in these regions using a lossy encoding (like JPEG).
-This can be really helpfull to save bandwidth when playing videos. Disabling
-adaptive encodings allow to restore the original static behavior of encodings
+This can be really helpful to save bandwidth when playing videos. Disabling
+adaptive encodings allows to restore the original static behavior of encodings
 like Tight.
 
 @end table

savevm.c

@@ -1008,7 +1008,7 @@ const VMStateInfo vmstate_info_buffer = {
 };
 
 /* unused buffers: space that was used for some fields that are
-   not usefull anymore */
+   not useful anymore */
 
 static int get_unused_buffer(QEMUFile *f, void *pv, size_t size)
 {

target-arm/helper.c

@@ -1378,7 +1378,7 @@ void HELPER(set_cp15)(CPUState *env, uint32_t insn, uint32_t val)
             /* This may enable/disable the MMU, so do a TLB flush.  */
             tlb_flush(env, 1);
             break;
-        case 1: /* Auxiliary cotrol register.  */
+        case 1: /* Auxiliary control register.  */
             if (arm_feature(env, ARM_FEATURE_XSCALE)) {
                 env->cp15.c1_xscaleauxcr = val;
                 break;