libvirt doesn't show errors when deploying a virtual machine on standalone.

[eroussy]

Describe the bug
When deploying a virtual machine with ansible, if an error occured in libvirt, the error message is not shown by ansible. Instead, it say msg: '''libvirtError'' object has no attribute ''message'''

To Reproduce
Steps to reproduce the behavior:

1. On a Yocto machine (standalone)
2. Create the inventory vm.yml (see below)
3. Launch the playbook deploy_vm_standalone.yaml to deploy the vm.
4. An error occured during the step "Create VMs"
5. The log of the error is incorrect : "msg: '''libvirtError'' object has no attribute ''message'''"

Expected behavior
In such situation, the correct libvirt error is visible when running journalctl -u libvirtd on the hypervisor :
libvirtd[627]: missing target information for device /var/lib/libvirt/images/guest.qcow2
This error should be visible in ansible logs.

I think I already see that also on a debian version, but I wasn't able to reproduce.
In cluster mode, it seems that a check is applied on xml before creating the virtual machines and this check catch the bug.
See if it is possible to run the check on a standalone machine too ?

The inventory vm.yaml

all:
  children:
    VMs:
      hosts:
        guest:
          ansible_host: 192.168.213.130
          vm_template: "../guest_false.xml"
          vm_disk: "../vm_images/guest.qcow2"
          vm_features: []

The associated xml "guest_false.xml" :

<?xml version="1.0" encoding="UTF-8"?>
<domain type="kvm">
    <name>{{ vm.inventory_hostname }}</name>
    <vcpu placement="static">1</vcpu>
    <memory unit="MiB">2048</memory>
    <currentMemory unit="MiB">2048</currentMemory>
    <os firmware="efi">
        <type arch="x86_64" machine="pc-i440fx-4.1">hvm</type>
        <boot dev="hd" />
        <bootmenu enable="no" />
        <bios useserial="yes" rebootTimeout="0" />
        <smbios mode="emulate" />
    </os>
    <features>
        <acpi />
        <apic />
        <vmport state="off" />
    </features>
    <cputune>
    </cputune>
    <cpu mode="custom" match="exact" check="none">
        <model fallback="allow">kvm64</model>
    </cpu>
    <clock offset="utc">
        <timer name="rtc" tickpolicy="catchup" />
        <timer name="pit" tickpolicy="delay" />
        <timer name="hpet" present="no" />
    </clock>
    <on_poweroff>destroy</on_poweroff>
    <on_reboot>restart</on_reboot>
    <on_crash>destroy</on_crash>
    <pm>
        <suspend-to-mem enabled="no" />
        <suspend-to-disk enabled="no" />
    </pm>
    <devices>
        <emulator>/usr/bin/qemu-system-x86_64</emulator>
        <disk type="file" device="disk">
           <driver name="qemu" type="qcow2"/>
           <source file="/var/lib/libvirt/images/guest.qcow2"/>
   <!--
           <target dev="vda" bus="virtio"/>
           When the target line is missing, libvirt will fail to find the disk
   -->
        </disk>
        <controller type="pci" index="0" model="pci-root" />
        <serial type="pty">
            <target type="isa-serial" port="0">
                <model name="isa-serial" />
            </target>
        </serial>
        <console type="pty">
            <target type="serial" port="0" />
        </console>
        <memballoon model="none" />
        <watchdog model="ib700" action="poweroff" />
    </devices>
</domain>

The error I injected is the missing target info in the disk part.