Trying to create VM fails silently

Do you have a block like this in /etc/config/virt?

config storage 'lvm'
        option driver 'lvm'
        option device 'vg1'

This would normally be created by muvirt-system-setup

If you don’t have an LVM or don’t want to use one, you could use an existing disk (like qcow2) and the ‘Provision in place’ (provinplace) option

config vm 'test'
    list disks '/mnt/storage/testvm.qcow2'
    option provinplace '1'

Rockstor also requires all the disks to have a serial set, like so:

config vm 'rockstor'
        list disks '/dev/mapper/vg1-rockstor408,serial=rroot'
        list disks '/dev/sda,serial=disk1'
        list disks '/dev/sdb,serial=disk2'

Hope that helps!