qemu-iotests: Fix _filter_qemu

$QEMU_PROG happens to be 'qemu' in my setup, so this sed command
replaces a bit too much. Restrict it to the start of the line and to
when it's followed by a colon, i.e. the form used by error messages.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>

tests/qemu-iotests/common.filter

@@ -155,7 +155,7 @@ _filter_qemu_io()
 # replace occurrences of QEMU_PROG with "qemu"
 _filter_qemu()
 {
-    sed -e "s#$(basename $QEMU_PROG)#QEMU_PROG#g"
+    sed -e "s#^$(basename $QEMU_PROG):#QEMU_PROG:#"
 }
 
 # make sure this script returns success