qemu-iotests: refine common.config

Replacing awk with sed, then it's easier to read.
Replacing "[ ! -z "$default_alias_machine" ]" with
"[[ $default_alias_machine ]]", then it's slightly shorter.
Reviewed-by: NMax Reitz <mreitz@redhat.com>
Suggested-By: NSascha Silbe <silbe@linux.vnet.ibm.com>
Reviewed-by: NSascha Silbe <silbe@linux.vnet.ibm.com>
Reviewed-by: NEric Blake   <eblake@redhat.com>
Signed-off-by: NBo Tu <tubo@linux.vnet.ibm.com>
Message-id: 1449136891-26850-2-git-send-email-tubo@linux.vnet.ibm.com
Signed-off-by: NMax Reitz <mreitz@redhat.com>

tests/qemu-iotests/common.config

@@ -154,11 +154,10 @@ export QEMU_IMG=_qemu_img_wrapper
 export QEMU_IO=_qemu_io_wrapper
 export QEMU_NBD=_qemu_nbd_wrapper
 
-default_machine=$($QEMU -machine \? | awk '/(default)/{print $1}')
-default_alias_machine=$($QEMU -machine \? |\
-    awk -v var_default_machine="$default_machine"\)\
-    '{if ($(NF-2)=="(alias"&&$(NF-1)=="of"&&$(NF)==var_default_machine){print $1}}')
-if [ ! -z "$default_alias_machine" ]; then
+default_machine=$($QEMU -machine help | sed -n '/(default)/ s/ .*//p')
+default_alias_machine=$($QEMU -machine help | \
+   sed -n "/(alias of $default_machine)/ { s/ .*//p; q; }")
+if [[ "$default_alias_machine" ]]; then
     default_machine="$default_alias_machine"
 fi