makebuildserver does not handle basebox with multiple architectures correctly

Given we have this boxes added to vagrant:

$ vagrant box list
fdroid-jessie64                 (libvirt, 0)
fdroid-jessie64                 (virtualbox, 0)

and configured makebuildserver.config.py like this:

basebox = "fdroid-stretch64"
vm_provider = 'libvirt'

makbuildserver will crash because the logic determining whether the box needs mutating is flawed.