Comment 4 for bug 1759806

Yes, the same thing has been happening to me since the Bionic upgrade:

Terminal warning is the same:
(gnome-boxes:2678): Boxes-WARNING **: 00:48:02.973: machine.vala:629: Failed to start Win10 Home: Unable to start domain: unsupported configuration: CPU mode 'custom' for x86_64 kvm domain on x86_64 host is not supported by hypervisor

(gnome-boxes:2678): Boxes-WARNING **: 00:48:34.093: machine.vala:629: Failed to start Fedora 27: Unable to start domain: unsupported configuration: CPU mode 'custom' for x86_64 kvm domain on x86_64 host is not supported by hypervisor

Virt-manager gives me this:
Error starting domain: unsupported configuration: CPU mode 'custom' for x86_64 kvm domain on x86_64 host is not supported by hypervisor

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: unsupported configuration: CPU mode 'custom' for x86_64 kvm domain on x86_64 host is not supported by hypervisor

All working fine before upgrade.