qemu: dump: Report better error when dumping VM with passthrough devices

qemu: dump: Report better error when dumping VM with passthrough devices

For the regular dump operation we migrate the VM to a file. This won't
work when the VM has passthrough devices assigned. Rather than reporting
a cryptic error from qemu run our check whether it can be migrated.

This does not influence the memory-only dump that is allowed with
passthrough devices.

Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=874418