iotests: Drop use of bash keyword 'function'

Bash allows functions to be declared with or without the leading
keyword 'function'; but including the keyword does not comply with
POSIX syntax, and is confusing to ksh users where the use of the
keyword changes the scoping rules for functions.  Stick to the
POSIX form through iotests.

Done mechanically with:
  sed -i 's/^function //' $(git ls-files tests/qemu-iotests)

Signed-off-by: Eric Blake <eblake@redhat.com>
Message-Id: <20181116215002.2124581-1-eblake@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>

tests/qemu-iotests/038

@@ -51,7 +51,7 @@ TEST_IMG="$TEST_IMG.base"
 
 _make_test_img $size
 
-function backing_io()
+backing_io()
 {
     local offset=$1
     local sectors=$2
@@ -76,7 +76,7 @@ _make_test_img -b "$TEST_IMG.base" 6G
 echo
 echo "== Some concurrent requests touching the same cluster =="
 
-function overlay_io()
+overlay_io()
 {
     # Start with a request touching two clusters
     echo aio_write -P 0x80 2020k 80k
@@ -102,7 +102,7 @@ overlay_io | $QEMU_IO "$TEST_IMG" | _filter_qemu_io |\
 echo
 echo "== Verify image content =="
 
-function verify_io()
+verify_io()
 {
     echo read -P 31 2016k 4k
     echo read -P 0x80 2020k 80k