Description of problem:

I was running virt-df [the --csv option is not important].  Since one
of the disks was inaccessible, I got an error.  That's not a problem,
but the fact that virt-df *hangs* after this error *is* a problem:

$ virt-df --csv
Virtual Machine,Filesystem,1K-blocks,Used,Available,Use%
"Debian5x64","/dev/debian5x64/home",3555936,1264800,2110504,35.6%
"Debian5x64","/dev/debian5x64/root",329233,200395,111840,60.9%
"Debian5x64","/dev/debian5x64/tmp",309401,10294,283133,3.3%
"Debian5x64","/dev/debian5x64/usr",3539776,1434740,1925224,40.5%
"Debian5x64","/dev/debian5x64/var",1741648,264632,1388544,15.2%
"Debian5x64","/dev/vda1",233335,47272,173615,20.3%
qemu: could not open disk image /dev/vg_trick/RHEL620100329n0x64: Permission denied

(eventually you have to hit ^C)

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

1.0.89

How reproducible:

Always.

Steps to Reproduce:
1. Run virt-df as non-root.
  
Actual results:

If there are any inaccessible disks, virt-df will hang.

Expected results:

Should give an error, but not hang afterwards.

Additional info:

Basically this is a problem that libguestfs does not properly
handle the case where the qemu process dies before connecting
back to the libguestfs socket (in the "null vmchannel" mode).