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>
diff --git a/tests/qemu-iotests/186.out b/tests/qemu-iotests/186.out
index b8bf9a2..c8377fe 100644
--- a/tests/qemu-iotests/186.out
+++ b/tests/qemu-iotests/186.out
@@ -442,28 +442,28 @@
     Cache mode:       writeback
 (qemu) quit
 
-qemu-system-x86_64: -drive if=scsi,driver=null-co: warning: bus=0,unit=0 is deprecated with this machine type
 Testing: -drive if=scsi,driver=null-co
 QEMU X.Y.Z monitor - type 'help' for more information
-(qemu) info block
+(qemu) QEMU_PROG: -drive if=scsi,driver=null-co: warning: bus=0,unit=0 is deprecated with this machine type
+info block
 scsi0-hd0 (NODE_NAME): null-co:// (null-co)
     Attached to:      /machine/unattached/device[27]/scsi.0/legacy[0]
     Cache mode:       writeback
 (qemu) quit
 
-qemu-system-x86_64: -drive if=scsi,media=cdrom: warning: bus=0,unit=0 is deprecated with this machine type
 Testing: -drive if=scsi,media=cdrom
 QEMU X.Y.Z monitor - type 'help' for more information
-(qemu) info block
+(qemu) QEMU_PROG: -drive if=scsi,media=cdrom: warning: bus=0,unit=0 is deprecated with this machine type
+info block
 scsi0-cd0: [not inserted]
     Attached to:      /machine/unattached/device[27]/scsi.0/legacy[0]
     Removable device: not locked, tray closed
 (qemu) quit
 
-qemu-system-x86_64: -drive if=scsi,driver=null-co,media=cdrom: warning: bus=0,unit=0 is deprecated with this machine type
 Testing: -drive if=scsi,driver=null-co,media=cdrom
 QEMU X.Y.Z monitor - type 'help' for more information
-(qemu) info block
+(qemu) QEMU_PROG: -drive if=scsi,driver=null-co,media=cdrom: warning: bus=0,unit=0 is deprecated with this machine type
+info block
 scsi0-cd0 (NODE_NAME): null-co:// (null-co, read-only)
     Attached to:      /machine/unattached/device[27]/scsi.0/legacy[0]
     Removable device: not locked, tray closed