[v2v] Fail migration if no conversion host configured

[bthurber]

BZ link: https://bugzilla.redhat.com/show_bug.cgi?id=1640816

Issue:
When initiating a migration plan, if there are no conversion hosts available the migration plan spins until the 40K timeout has been reached.

Resolution:
If there is no conversion host available fail the migration and provide a reason why in the UI.

[kedark3]

👍

[michaelkro]

Case where there are no configured conversion hosts. The request will be denied.

[mturley]

I think this is a duplicate of #786 (or that is a duplicate of this). Since we've referenced #786 in my WIP PR (#798) and elsewhere already, I'll put the BZ link over there and mark the PR as closing both issues.