18.04: VM stalls at initramfs due to not finding the rootfs block device

[TJ]

$ vagrant up tactyle
...
==> tactyle: Creating shared folders metadata...
==> tactyle: Starting domain.
==> tactyle: Waiting for domain to get an IP address...

And it stalls there.

Using Virtual Machine Manager GUI I was able to monitor what the vagrant-libvirt VM was doing and found it had dropped to the initramfs shell.

Using 'blkid' and also checking /sys/block/ and /dev/block/ revealed there were no storage devices (only loop device nodes).

Looking at the device config revealed the "Controller SCSI" was using the type "Hypervisor default" instead of "VirtIO SCSI". Changing that manually via Virtual Machine Manager and forcing a reboot solved the issue.

Currently vagrant-libvirt does not provide a way to set the device type. It seems that the QEMU default may have changed relatively recently to cause this.

The relevant differences in the libvirt domain XML are:

     <disk type='file' device='disk'>
       <driver name='qemu' type='qcow2'/>
       <source file='/var/lib/libvirt/images/tactyle_tactyle.img'/>
- <target dev='vda' bus='virtio'/>
- <address type='pci' domain='0x0000' bus='0x00' slot='0x07' function='0x0'/>
+ <target dev='vda' bus='scsi'/>
+ <address type='drive' controller='0' bus='0' target='0' unit='0'/>
     </disk>
     <controller type='pci' index='0' model='pci-root'/>
- <controller type='scsi' index='0'>
+ <controller type='scsi' index='0' model='virtio-scsi'>
     </controller>