Don't check for no VBDs at all

We just need to check for there being no VBDs owned by resident VMs.

Signed-off-by: John Else <john.else@citrix.com>
Fix error reporting

Signed-off-by: John Else <john.else@citrix.com>
Ignore tapdisks corresponding to ISOs

Signed-off-by: John Else <john.else@citrix.com>