Description of problem:

Should improve error info when can't find backing file for snapshot.

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

Host A:rhel6.4.z
# uname -r
2.6.32-358.27.1.el6.x86_64
# rpm -q qemu-kvm-rhev
qemu-kvm-rhev-0.12.1.2-2.355.el6_4.9.x86_64

Host B:rhel6.5
# uname -r
2.6.32-429.el6.x86_64
# rpm -q qemu-kvm-rhev
qemu-kvm-rhev-0.12.1.2-2.415.el6.x86_64

Guest:
2.6.32-428.el6.i686


How reproducible:

100%

Preparation:NFS

HostA:
#vi /etc/exports 
/home *(rw,no_root_squash)

#service nfs start
#mount -o soft,timeo=600,retrans=2,nosharecache 10.66.84.13:/home/ /mnt

HostB:

#mount -o soft,timeo=600,retrans=2,nosharecache 10.66.84.13:/home/ /mnt


Steps to Reproduce:

1.Boot guest on hostA

2.Create snapshot chains, 
 Base image -> snapshot A -> snapshot B -> snapshot C 

3.On hostA
#cd /mnt
#ll
-rw-r--r--. 1 root root 21474836480 Nov  8 13:06 rhel6-instal6.4-zhost.raw
-rw-r--r--. 1 root root    26083328 Nov  7 17:15 snapshotA
-rw-r--r--. 1 root root    25821184 Nov  7 17:15 snapshotB
-rw-r--r--. 1 root root    28114944 Nov  7 17:57 snapshotC

# qemu-img info snapshotC
image: snapshotC.back
file format: qcow2
virtual size: 20G (21474836480 bytes)
disk size: 27M
cluster_size: 65536
backing file: /home/snapshotB

3.Shutdown guest on HostA,then boot snapshotC on  HostB

... -drive file=/mnt/snapshotC,if=none,media=cdrom,id=drive-ide0-1-0,readonly=on,format=qcow2 -device ide-drive,bus=ide.1,unit=1,drive=drive-ide0-1-0,id=ide0-1-0 
...


Actual results:
Error info:
qemu-kvm: -drive file=/mnt/snapshotC,if=none,media=cdrom,id=drive-ide0-1-0,readonly=on,format=qcow2: could not open disk image /mnt/snapshotC: No such file or directory


Expected results:

Should give the error info about can't find the backing file.

EG:
qemu-kvm:-drive file=/mnt/snapshotC,if=none,media=cdrom,id=drive-ide0-1-0,readonly=on,format=qcow2:could not find the backing file /home/snapshotB.


Additional info:


My CLI:
HOSTB:
 /usr/libexec/qemu-kvm -cpu Opteron_G3 -M rhel6.4.0 -enable-kvm -m 4096 -smp 2,cores=1,threads=1 -name RHEL6.5 -uuid `uuidgen` -nodefconfig -nodefaults -monitor stdio -rtc base=utc -device piix3-usb-uhci,id=usb,bus=pci.0,addr=0x1.0x2 -device virtio-serial-pci,id=virtio-serial0,bus=pci.0,addr=0x7 -device virtio-scsi-pci,bus=pci.0,addr=0x5,id=scsi0 -drive file=/mnt/snapshotC,if=none,media=cdrom,id=drive-ide0-1-0,readonly=on,format=qcow2 -device ide-drive,bus=ide.1,unit=1,drive=drive-ide0-1-0,id=ide0-1-0 -device usb-tablet,id=input0 -device intel-hda,id=sound0,bus=pci.0,addr=0x4 -device hda-duplex,id=sound0-codec0,bus=sound0.0,cad=0 -device virtio-balloon-pci,id=balloon0,bus=pci.0,addr=0x6 -netdev tap,id=hostnet2,fd=20 -device virtio-net-pci,__com_redhat_macvtap_compat=on,netdev=hostnet2,id=net0,mac=00:10:20:2d:31:21,addr=0x8 20<>/dev/tap4 -vnc :10 -vga qxl -global PIIX4_PM.disable_s3=0 -global PIIX4_PM.disable_s4=0 -boot menu=on