Description of problem:
qemu abort when live snapshot for multiple block device simultaneously with transaction and one is to a non-exist path.

Version-Release number of selected component (if applicable):


How reproducible:


Steps to Reproduce:
1. Launch guest with two block device.

# gdb /usr/libexec/qemu-kvm

(gdb)  run -drive file=/home/rhel74-64-virtio-scsi.qcow2,format=qcow2,if=none,id=drive-scsi-disk0,werror=stop,rerror=stop,cache=none -device virtio-scsi-pci,id=scsi0 -device scsi-hd,drive=drive-scsi-disk0,bus=scsi0.0,scsi-id=0,lun=0,id=scsi-disk0 -monitor stdio -qmp tcp:0:5555,server,nowait -vnc :3 -netdev tap,id=hostnet0,script=/etc/qemu-ifup -device virtio-net-pci,netdev=hostnet0,id=virtio-net-pci0,mac=70:e2:84:14:0e:15 -drive file=/home/test.qcow2,format=qcow2,if=none,id=drive-scsi-disk1,werror=stop,rerror=stop,cache=none -device scsi-hd,drive=drive-scsi-disk1,bus=scsi0.0,scsi-id=1,lun=1,id=scsi-disk1

2. live snapshot simultaneously with transaction, "/home/test/test/test" below does not exist.
{ "execute": "transaction", "arguments": { "actions": [ { 'type': 'blockdev-snapshot-sync', 'data' : { "device": "drive-scsi-disk0","snapshot-file": "/home/sn1-a.qcow2","mode": "absolute-paths", "format": "qcow2" } }, { 'type': 'blockdev-snapshot-sync', 'data' : { "device": "drive-scsi-disk1","snapshot-file": "/home/test/test/test","mode": "absolute-paths", "format": "qcow2" } }]}}


Actual results:
qemu abort.
(qemu) [Thread 0x3fffb47eea80 (LWP 11890) exited]
Formatting '/home/sn1-a.qcow2', fmt=qcow2 size=21474836480 backing_file=/home/rhel74-64-virtio-scsi.qcow2 backing_fmt=qcow2 encryption=off cluster_size=65536 lazy_refcounts=off refcount_bits=16
[New Thread 0x3fffb47eea80 (LWP 11968)]
Formatting '/home/test/test/test', fmt=qcow2 size=21474836480 backing_file=/home/test.qcow2 backing_fmt=qcow2 encryption=off cluster_size=65536 lazy_refcounts=off refcount_bits=16
Unexpected error in bdrv_check_update_perm() at block.c:1611:
Conflicts with use by drive-scsi-disk0 as 'backing', which does not allow 'write' on #block198

Program received signal SIGABRT, Aborted.
0x00003fffb6f2ef90 in __GI_raise (sig=<optimized out>) at ../nptl/sysdeps/unix/sysv/linux/raise.c:56
56	  return INLINE_SYSCALL (tgkill, 3, pid, selftid, sig);

Expected results:
qmp prompt error like:
{"error": {"class": "GenericError", "desc": "Could not create file: No such file or directory"}}


Additional info:
1. This issue is ppc only, x86 works well.
2. qmp prompts error as expect and qemu works normally if replace step 2 with either:
{ "execute": 'blockdev-snapshot-sync', "arguments": { "device": "drive-scsi-disk1","snapshot-file": "/home/test/test/test","mode": "absolute-paths", "format": "qcow2" } }
or:
{ "execute": "transaction", "arguments": { "actions": [ { 'type': 'blockdev-snapshot-sync', 'data' : { "device": "drive-scsi-disk1","snapshot-file": "/home/test/test/test","mode": "absolute-paths", "format": "qcow2" } }]}}