Description of problem:
cannot open Packages index using db5 - Resource temporarily unavailable

Version-Release number of selected component (if applicable):
virt-v2v-1.42.0-18.module+el8.6.0+13447+4b5d0856.x86_64
libguestfs-1.44.0-5.module+el8.6.0+13732+b2b9b31d.x86_64
rpm-4.14.3-20.el8.x86_64

How reproducible:
100%

Steps to Reproduce:
1. Put a image file into a NFS server and mount it to local dir.
2. Run virt-v2v command to convert the disk.
# ll v2v/Auto-kvm-rhel7.1-sparseqcow2.img
-rw-r--r--. 1 nobody nobody 3936354304 Jun 19  2015 v2v/Auto-kvm-rhel7.1-sparseqcow2.img
# virt-v2v -i disk v2v/Auto-kvm-rhel7.1-sparseqcow2.img -o null
[   0.0] Opening the source -i disk v2v/Auto-kvm-rhel7.1-sparseqcow2.img
[   0.1] Creating an overlay to protect the source from being modified
[   0.2] Opening the overlay
[   5.5] Inspecting the overlay
[  45.2] Checking for sufficient free disk space in the guest
[  45.2] Estimating space required on target for each disk
[  45.2] Converting Red Hat Enterprise Linux Server 7.1 (Maipo) to run on KVM
virt-v2v: error: no installed kernel packages were found.

This probably indicates that virt-v2v was unable to inspect this guest 
properly.

If reporting bugs, run virt-v2v with debugging enabled and include the 
complete output:

  virt-v2v -v -x [...]
3. Check the v2v debug log(should enable it by '-v -x').


Actual results:
libguestfs: trace: v2v: inspect_get_package_format = "rpm"
libguestfs: trace: v2v: internal_list_rpm_applications
guestfsd: => inspect_get_package_format (0x1e5) took 0.01 secs^M
guestfsd: <= internal_list_rpm_applications (0x1fe) request length 40 bytes^M
command: mount returned 0^M
chroot: /sysroot: running 'librpm'^M
\x1b[31;1merror: \x1b[0m\x1b[1mdb5 error(11) from dbenv->open: Resource temporarily unavailable^M
\x1b[0m\x1b[31;1merror: \x1b[0m\x1b[1mcannot open Packages index using db5 - Resource temporarily unavailable (11)^M
\x1b[0m\x1b[31;1merror: \x1b[0m\x1b[1mcannot open Packages database in ^M
\x1b[0m\x1b[31;1merror: \x1b[0m\x1b[1mdb5 error(11) from dbenv->open: Resource temporarily unavailable^M
\x1b[0m\x1b[31;1merror: \x1b[0m\x1b[1mcannot open Packages index using db5 - Resource temporarily unavailable (11)^M
\x1b[0m\x1b[31;1merror: \x1b[0m\x1b[1mcannot open Packages database in ^M
\x1b[0mlibrpm returned 0 installed packages^M

Expected results:
The librpm should be called successfully.

Additional info:
1) I can't reproduce it by other images in the same NFS directory. Only 'Auto-kvm-rhel7.1-sparseqcow2.img' has problem. When I first met this issue, after I copied it from NFS to a local directory, I couldn't reproduce it. But now the same step can also reproduce it. I'm not sure what's the difference. 
Now that it can be reproduced after copying it to a local directory from NFS, I created a VM by virt-install by importing the disk and the system seems fine. then power-off it and try the v2v command again, the issue is gone.
But it can still be reproduce if recopying a new image or run it directly in NFS directory. 
Maybe the rpmdb files in the guest has problem? and they are recovered after a new boot?
2) Another similar scenario can generate same error.
   2.1 Remove the rpm db fiels. 
       # rm /var/lib/rpm/__db.*
   2.2 Create a blank file instead. 
       # touch /var/lib/rpm/__db.001
   2.3 poweroff the guest, and convert it by v2v.
   2.4 The same error is reported.