iotests: Redirect stderr to stdout in 186

iotests: Redirect stderr to stdout in 186

Without redirecting qemu's stderr to stdout, _filter_qemu will not apply
to warnings.  This results in $QEMU_PROG not being replaced by QEMU_PROG
which is not great if your qemu executable is not called
qemu-system-x86_64 (e.g. qemu-system-i386).

Signed-off-by: Max Reitz <mreitz@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Jeff Cody <jcody@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>