Packer/QEMU/SUSE(OS) - Unable to unmount /var/lib/nfs/rpc_pipefs

_This issue was originally opened by @ry1015 as hashicorp/packer#10284. It was migrated here as a result of the [Packer plugin split](https://github.com/hashicorp/packer/issues/8610#issuecomment-770034737). The original body of the issue is below._

Hi,

The installation gets stuck on the unmounting phase specifically trying to unmount /var/lib/nfs/rpc_pipefs. I've attached our json config, screenshot of the error and the packer log. I've also tried different memory sizes to no avail. I've also tried virtio-scsi, scsi for the disk-interface option.

packer version: 1.6.0

JSON CONFIG
`{
"provisioners": [],
"variables": {},
"builders": [
{
"boot_wait": "14s",
"disk_size": 60000,
"headless": "true",
"http_directory": "./config/sles",
"iso_checksum": "633537da81d270a9548272dfe1fdd20d",
"iso_url": "/ISOs/SLE-12-SP3-Server-DVD-x86_64-GM-DVD1.iso",
"output_directory": "output-sles12sp3qemu",
"qemuargs": [
[ "-m", "4048M" ]
],
"type": "qemu",
"vm_name": "sles12sp3",
"ssh_username": "test",
"ssh_password": "testme",
"ssh_wait_timeout": "1200s",
"format": "raw",
"boot_command": [
"linux netsetup=dhcp ",
"lang=en_US autoyast=http://{{ .HTTPIP }}:{{ .HTTPPort }}/autoyast_sles12.xml install=cd:/ ",
"textmode=1",
""
],
"disk_interface": "ide"
}
],
"sensitive-variables": [
"ssh_pass"
]
}`

SCREENSHOT

PACKER LOG
[packer_log.txt](https://github.com/hashicorp/packer/files/5562966/packer_log.txt)

Thanks in advance,
Ry

Start by reviewing the attached Packer log, screenshot, and JSON configuration for the SLES 12 SP3 QEMU build, focusing on the attempted unmount of /var/lib/nfs/rpc_pipefs. Reproduce the installation with the listed settings and determine what prevents the build from completing; done means the unmount phase finishes successfully.

Written by the indexing model from the issue text.