Verified with package:
libguestfs-1.38.2-12.el7_6.2.x86_64

Steps:

1. Download the condor-fedora-24-64-master-2016080801.qcow2 image from the link: https://platform.swampinabox.org/platform-images/condor-fedora-24-64-master-2016080801.qcow2.gz
2. 
# virt-inspector -a condor-fedora-24-64-master-2016080801.qcow2
libguestfs: error: mount_ro: mount_ro_stub: /dev/vdb: expecting a device name
libguestfs: error: mount_ro: mount_ro_stub: /dev/vdc: expecting a device name
virt-inspector: some filesystems could not be mounted (ignored)
<?xml version="1.0"?>
<operatingsystems>
  <operatingsystem>
    <root>/dev/vg0/lv_root</root>
    <name>linux</name>
    <arch>x86_64</arch>
    <distro>fedora</distro>
    <product_name>Fedora 24 (Twenty Four)</product_name>
    <major_version>24</major_version>
    <minor_version>0</minor_version>
    <package_format>rpm</package_format>
    <package_management>dnf</package_management>
    <hostname>localhost.localdomain</hostname>
    <osinfo>fedora24</osinfo>
    <mountpoints>
      <mountpoint dev="/dev/vg0/lv_root">/</mountpoint>
      <mountpoint dev="/dev/sda2">/boot</mountpoint>
      <mountpoint dev="/dev/sdb">/mnt/in</mountpoint>
      <mountpoint dev="/dev/sdc">/mnt/out</mountpoint>
    </mountpoints>
    <filesystems>
      <filesystem dev="/dev/sda2">
        <type>ext4</type>
        <uuid>4c029e08-3e60-4227-8a5b-de2c500daf08</uuid>
      </filesystem>
      <filesystem dev="/dev/sdb"/>
      <filesystem dev="/dev/sdc"/>
      <filesystem dev="/dev/vg0/lv_root">
        <type>ext4</type>
        <uuid>36ef4986-e9a4-4177-b251-455618165972</uuid>
      </filesystem>
      <filesystem dev="/dev/vg0/lv_swap">
        <type>swap</type>
        <uuid>299e4b2f-4946-4b48-97ce-6b6afd4735e7</uuid>
      </filesystem>
    </filesystems>
...

The virt-inspector command can inspect the linux image when /etc/fstab contains a partionless device. Verified this bug.

Reproduce the bug with builds:
virt-v2v-1.38.2-12.el7_6.1.x86_64
libguestfs-1.38.2-12.el7_6.1.x86_64

Steps to reproduce:
1.Add one disk to a linux guest on VMware host, then create partition for the new added disk and format the disk as ext3

2.Create /mnt/in and add below info in /etc/fstab (the new disk has no partition number)
# cat /etc/fstab
....
/dev/mapper/rhel-root   /                       xfs     defaults        0 0
UUID=3c069c5d-9497-4d39-808c-011354d29760 /boot                   xfs     defaults        0 0
/dev/mapper/rhel-swap   swap                    swap    defaults        0 0
/dev/sdb        /mnt/in                ext3        defaults        0 0
      

3.Reboot the guest, guest will boot into emergency mode

4.Power off guest, then use virt-v2v to convert it to rhv
# virt-v2v -ic vpx://root.73.141/data/10.73.75.219/?no_verify=1  --password-file /tmp/passwd esx6.7-rhel7.6-x86_64-fstab-partionless -o rhv -os 10.66.144.40:/home/nfs_export
[   0.0] Opening the source -i libvirt -ic vpx://root.73.141/data/10.73.75.219/?no_verify=1 esx6.7-rhel7.6-x86_64-fstab-partionless
[   3.9] Creating an overlay to protect the source from being modified
[   5.3] Initializing the target -o rhv -os 10.66.144.40:/home/nfs_export
[   5.6] Opening the overlay
[  45.4] Inspecting the overlay
virt-v2v: error: libguestfs error: inspect_os: int_of_string

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

  virt-v2v -v -x [...]


Verify the bug with builds:
virt-v2v-1.38.2-12.el7_6.2.x86_64
libguestfs-1.38.2-12.el7_6.2.x86_64

Steps:
1.Convert above guest by virt-v2v again
# virt-v2v -ic vpx://root.73.141/data/10.73.75.219/?no_verify=1  --password-file /tmp/passwd esx6.7-rhel7.6-x86_64-fstab-partionless -o rhv -os 10.66.144.40:/home/nfs_export
[   0.0] Opening the source -i libvirt -ic vpx://root.73.141/data/10.73.75.219/?no_verify=1 esx6.7-rhel7.6-x86_64-fstab-partionless
[   4.2] Creating an overlay to protect the source from being modified
[   5.7] Initializing the target -o rhv -os 10.66.144.40:/home/nfs_export
[   6.2] Opening the overlay
[  28.8] Inspecting the overlay
virt-v2v: warning: mount: mount exited with status 32: mount: /dev/sdb is 
write-protected, mounting read-only
mount: unknown filesystem type '(null)' (ignored)
[ 216.5] Checking for sufficient free disk space in the guest
[ 216.5] Estimating space required on target for each disk
[ 216.5] Converting Red Hat Enterprise Linux Server 7.6 (Maipo) to run on KVM
virt-v2v: This guest has virtio drivers installed.
[1443.4] Mapping filesystem data to avoid copying unused and blank areas
[1453.1] Closing the overlay
[1453.6] Checking if the guest needs BIOS or UEFI to boot
[1453.6] Assigning disks to buses
[1453.6] Copying disk 1/2 to /tmp/v2v.8vzNy7/c980b640-cb59-47f7-a0fe-90312d33deae/images/c9412985-5146-4db6-8fc5-7796771199d8/cf6212ac-7e65-452d-a81f-907aefdfd991 (raw)
    (100.00/100%)
[2094.6] Copying disk 2/2 to /tmp/v2v.8vzNy7/c980b640-cb59-47f7-a0fe-90312d33deae/images/ac4552f7-ad1f-4cdd-9543-52d8af036eb7/64d2aa7d-0761-4f8a-8166-0ebee5cbeb7c (raw)
    (100.00/100%)
[2176.4] Creating output metadata
[2176.5] Finishing off

2.After finishing conversion,import guest from export domain to data domain and power on, guest will boot into emergency mode

Since the problem described in this bug report should be
resolved in a recent advisory, it has been closed with a
resolution of ERRATA.

For information on the advisory, and where to find the updated
files, follow the link below.

If the solution does not work for you, open a new bug report.

https://access.redhat.com/errata/RHBA-2019:0513