Description of problem:
'virsh vol-dumpxml' failed to get vol info for images created with host_cdrom json backing when the cdrom is empty

Version-Release number of selected component (if applicable):
libvirt-4.4.0-2.el7.x86_64

How reproducible:
100%

Steps to Reproduce:
1.Create backing file with host_cdrom json backing and the cdrom is empty:
# qemu-img create -f qcow2 -b 'json:{"file.driver":"host_cdrom", "file.filename":"/dev/sr0"}' /var/lib/libvirt/images/host_cdrom.img
Formatting '/var/lib/libvirt/images/host_cdrom.img', fmt=qcow2 size=1073741312 backing_file=json:{"file.driver":"host_cdrom",, "file.filename":"/dev/sr0"} cluster_size=65536 lazy_refcounts=off refcount_bits=16

2.Refresh the pool:
#virsh pool-refresh default
Pool default refreshed

3.Execute 'virsh vol-dumpxml':
#virsh vol-dumpxml host_cdrom.img default
error: cannot read header '/dev/sr0': Input/output error


Actual results:
'virsh vol-dumpxml' failed to get vol info for images created with host_cdrom json backing when the cdrom is empty.


Expected results:
'virsh vol-dumpxml' should work well for images created with host cdrom json backing

Additional info:
1.Can get images info successfully with 'qemu-img info'
# qemu-img info /var/lib/libvirt/images/host_cdrom.img
image: /var/lib/libvirt/images/host_cdrom.img
file format: qcow2
virtual size: 1.0G (1073741312 bytes)
disk size: 196K
cluster_size: 65536
backing file: json:{"file.driver":"host_cdrom", "file.filename":"/dev/sr0"}
Format specific information:
    compat: 1.1
    lazy refcounts: false
    refcount bits: 16
    corrupt: false