Description of problem:
In Fedora 19's libguestfs, qemu is started with cache=none when attempting to mount the new image in order to inject the configuration. This fails to pread the qcow2 image: pread(9, 0x7f1d44000a00, 512, 0) = -1 EINVAL (Invalid argument)

if the image is on xfs with sectsz=4096:
meta-data=/dev/mapper/fedora_ewcs9-var isize=256    agcount=49, agsize=319872 blks
         =                       sectsz=4096  attr=2
data     =                       bsize=4096   blocks=15360000, imaxpct=25
         =                       sunit=128    swidth=256 blks
naming   =version 2              bsize=4096   ascii-ci=0
log      =internal               bsize=4096   blocks=2560, version=2
         =                       sectsz=4096  sunit=1 blks, lazy-count=1
realtime =none                   extsz=4096   blocks=0, rtextents=0

Version-Release number of selected component (if applicable):
libguestfs-1.22.7-1.fc19

How reproducible:
Always

Steps to Reproduce:
Install with packstack.
1. mkfs -s 4096 /dev/vgfoo/lvbar
2. mount /dev/vgfoo/lvbar /var/lib/nova/instances
3. Install with packstack
4. start openstack
5. Install an image and launch a VM from that image

Actual results:
The VM is not started. A log entry about ignoring the failure to inject data into the image is logged.

An strace revealed that qemu is being called by libguestfs attempting to use -drive file={image filename},if=none,id=drive-scsi0-0-0-0,format=qcow2,cache=none

Attempting that same command results in the error, "Invalid argument"
A further strace reveals the EINVAL comes from pread due to the block size mismatch.

Expected results:
The data is injected and the VM starts.

Additional info:
This bug is fixed in libguestfs 1.24 because it uses writeback or unsafe depending on mode, and never uses cache=none