VirtioFS doesen't like the simlink to the true disk location that the new exclusive shares feature sets up. Unraid's VM manager automatically points to the simlink location instead of the disk location. The below changes basically bypass the (simlink) middleman and play nice with virtiofs.
You can resolve this without the NFS workaround by changing the <source dir=' '/> path in your VM's XML.
BEFORE
<filesystem type='mount' accessmode='passthrough'>
<driver type='virtiofs' queue='1024'/>
<binary path='/usr/libexec/virtiofsd' xattr='on'>
<cache mode='always'/>
<sandbox mode='chroot'/>
</binary>
<source dir='/mnt/user/some_share_name'/>
<target dir='vm_storage'/>
<address type='pci' domain='0x0000' bus='0x01' slot='0x00' function='0x0'/>
</filesystem>
AFTER
<filesystem type='mount' accessmode='passthrough'>
<driver type='virtiofs' queue='1024'/>
<binary path='/usr/libexec/virtiofsd' xattr='on'>
<cache mode='always'/>
<sandbox mode='chroot'/>
</binary>
<source dir='/mnt/cache_ssd/some_share_name'/>
<target dir='vm_storage'/>
<address type='pci' domain='0x0000' bus='0x01' slot='0x00' function='0x0'/>
</filesystem>