Description of problem:

Out company and others encountered the following sporadic qemu crash for VMs that are stored on NFS.

qemu-system-x86_64: block.c:2806: bdrv_error_action: Assertion `error >= 0' failed.
2014-06-30 08:40:11.686+0000: shutting down

Form reading the code at the mentioned location I assume there should be some
further details to find.

...
void bdrv_error_action(BlockDriverState *bs, BlockErrorAction action,
                       bool is_read, int error)
{
    assert(error >= 0);
    bdrv_emit_qmp_error_event(bs, QEVENT_BLOCK_IO_ERROR, action, is_read);
    if (action == BDRV_ACTION_STOP) {
        vm_stop(RUN_STATE_IO_ERROR);
        bdrv_iostatus_set_err(bs, error);
    }
}
...

Qemu is started with libvirt so these are the only logs I found. Any idea where to 
search next?

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

Fedora 20, qemu 1.6.2

How reproducible:

once per month

Steps to Reproduce:

Don't know

Actual results:

VM crashes

Expected results:

vm continues to run, a more helpful error is shown.