qemu-options: add missing -drive discard option to cmdline help

Signed-off-by: NPeter Lieven <pl@kamp.de>
Reviewed-by: NEric Blake <eblake@redhat.com>
Signed-off-by: NStefan Hajnoczi <stefanha@redhat.com>

qemu-options.hx

@@ -427,7 +427,7 @@ DEF("drive", HAS_ARG, QEMU_OPTION_drive,
     "       [,serial=s][,addr=A][,rerror=ignore|stop|report]\n"
     "       [,werror=ignore|stop|report|enospc][,id=name][,aio=threads|native]\n"
     "       [,readonly=on|off][,copy-on-read=on|off]\n"
-    "       [,detect-zeroes=on|off|unmap]\n"
+    "       [,discard=ignore|unmap][,detect-zeroes=on|off|unmap]\n"
     "       [[,bps=b]|[[,bps_rd=r][,bps_wr=w]]]\n"
     "       [[,iops=i]|[[,iops_rd=r][,iops_wr=w]]]\n"
     "       [[,bps_max=bm]|[[,bps_rd_max=rm][,bps_wr_max=wm]]]\n"