qemu-iotests: Discard stderr when probing devices

qemu-iotests fails in the following setup:

  ./configure --enable-modules --enable-smartcard \
      --target-list=x86_64-softmmu,s390x-softmmu
  make
  cd build
  QEMU_PROG=`pwd`/s390x-softmmu/qemu-system-s390x \
      ../tests/check-block.sh qcow2
  ...
  --- /home/crobinso/src/qemu/tests/qemu-iotests/127.out
  +++ /home/crobinso/src/qemu/build/tests/qemu-iotests/scratch/127.out.bad
  @@ -1,4 +1,18 @@
   QA output created by 127
  +Failed to open module: /home/crobinso/src/qemu/build/hw-usb-smartcard.so: undefined symbol: ccid_card_ccid_attach
  ...
  --- /home/crobinso/src/qemu/tests/qemu-iotests/267.out
  +++ /home/crobinso/src/qemu/build/tests/qemu-iotests/scratch/267.out.bad
  @@ -1,4 +1,11 @@
   QA output created by 267
  +Failed to open module: /home/crobinso/src/qemu/build/hw-usb-smartcard.so: undefined symbol: ccid_card_ccid_attach

The stderr spew is its own known issue, but seems like iotests should
be discarding stderr in this case.

Signed-off-by: Cole Robinson <crobinso@redhat.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

tests/qemu-iotests/common.rc

@@ -982,7 +982,7 @@ _require_large_file()
 #
 _require_devices()
 {
-    available=$($QEMU -M none -device help | \
+    available=$($QEMU -M none -device help 2> /dev/null | \
                 grep ^name | sed -e 's/^name "//' -e 's/".*$//')
     for device
     do
@@ -994,7 +994,7 @@ _require_devices()
 
 _require_one_device_of()
 {
-    available=$($QEMU -M none -device help | \
+    available=$($QEMU -M none -device help 2> /dev/null | \
                 grep ^name | sed -e 's/^name "//' -e 's/".*$//')
     for device
     do