Windows 10 host and Linux guest, can I swap the two license wise?

Its a bit tricky.

As per the EULA (Part 4b for those playing at home)

b. Stand-alone software. If you acquired the software as stand-alone software (and also if you upgraded from software you acquired as stand-alone software), you may transfer the software to another device that belongs to you. You may also transfer the software to a device owned by someone else if (i) you are the first licensed user of the software and (ii) the new user agrees to the terms of this agreement. You may use the backup copy we allow you to make or the media that the software came on to transfer the software. Every time you transfer the software to a new device, you must remove the software from the prior device. You may not transfer the software to share licenses between devices.

So, in theory yes. According to the official license documents, if you upgrade to Windows 10 from a Retail (transferrable) copy of Windows 8.x you retain the same Retail (transferrable) rights. This has been also been stated by Microsoft officials. source

In practice however, Microsoft's upgrade and licensing systems are incapable of actually handling this, so you only get a non-transferrable Windows 10 "Digital entitlement" for that individual machine. (source) (source)

A Windows 10 digital entitlement associated with your Microsoft account has no key. Digital entitlements are locked to the machine they were first installed on. Microsoft will only allow you to restore your digital entitlement from your Microsoft Account onto a machine of the same type and manufacturer as the original - because you have to convince it it's the "same machine". Therefore, you can't take an entitlement linked to your account from a Dell machine and restore it onto a Lenovo, or a VM.

So in practice, you would have the "right" to do what you propose, but in reality you can't. Microsoft seems to have no official workaround, instead issuing individuals with Retail Windows 10 keys on an ad-hoc basis if they complain to MS Support enough.

One way around this would be to use your windows 8 key with a windows 10 installer, though phone or chat activation may be needed with an explanation that you have completely deleted your old install. Alternatively, you can also install the VM initially with Windows 8.1 Pro, and then perform the "Free upgrade to Windows 10" procedure again, which still works. Microsoft also officially still give out free upgrades to those using "accessibility software" which is not actually defined or checked anywhere. (source)

I believe you can run windows in trial mode for a short period of time. This might be a good idea before you activate the licence on the VM - it ensures that the VM is good enough for what you want it to do, and is a reasonably safe way to test it.