On rebooting post os install will not start with unsafe cache option on empty cdrom drive for iso.

[Lyn Perrine]

To reproduce go to install any os in virt-manger I tried with Lubuntu 18.10 as a guest and before starting the vm click to customize hardware on the cd drive under performace option set unsafe for the cache mode. When the os reboots the first time to eject the os install iso from the cd drive I get the following error message.

Error starting domain: internal error: process exited while connecting to monitor: 2018-05-14T03:32:17.914538Z qemu-system-x86_64: -drive if=none,id=drive-ide0-0-1,readonly=on,cache=unsafe: Must specify either driver or file

Traceback (most recent call last):
  File "/usr/share/virt-manager/virtManager/asyncjob.py", line 89, in cb_wrapper
    callback(asyncjob, *args, **kwargs)
  File "/usr/share/virt-manager/virtManager/asyncjob.py", line 125, in tmpcb
    callback(*args, **kwargs)
  File "/usr/share/virt-manager/virtManager/libvirtobject.py", line 82, in newfn
    ret = fn(self, *args, **kwargs)
  File "/usr/share/virt-manager/virtManager/domain.py", line 1508, in startup
    self._backend.create()
  File "/usr/lib/python2.7/dist-packages/libvirt.py", line 1062, in create
    if ret == -1: raise libvirtError ('virDomainCreate() failed', dom=self)
libvirtError: internal error: process exited while connecting to monitor: 2018-05-14T03:32:17.914538Z qemu-system-x86_64: -drive if=none,id=drive-ide0-0-1,readonly=on,cache=unsafe: Must specify either driver or file

virt-manager:
  Installed: 1:1.5.1-0ubuntu1
  Candidate: 1:1.5.1-0ubuntu1
  Version table:
 *** 1:1.5.1-0ubuntu1 500
        500 http://us.archive.ubuntu.com/ubuntu bionic/universe amd64 Packages
        500 http://us.archive.ubuntu.com/ubuntu bionic/universe i386 Packages
        100 /var/lib/dpkg/status

Description: Ubuntu 18.04 LTS
Release: 18.04
I expected the os to reboot. Instead it does not reboot and pops up the error message seen earlier.

ProblemType: Bug
DistroRelease: Ubuntu 18.04
Package: virt-manager 1:1.5.1-0ubuntu1
ProcVersionSignature: Ubuntu 4.15.0-20.21-generic 4.15.17
Uname: Linux 4.15.0-20-generic x86_64
ApportVersion: 2.20.9-0ubuntu7
Architecture: amd64
CurrentDesktop: LXDE
Date: Sun May 13 20:32:37 2018
InstallationDate: Installed on 2018-03-13 (62 days ago)
InstallationMedia: Lubuntu 16.04.4 LTS "Xenial Xerus" - Release amd64 (20180226)
PackageArchitecture: all
SourcePackage: virt-manager
UpgradeStatus: Upgraded to bionic on 2018-04-12 (31 days ago)