travis: turn off loop-device & device-mapper on x86_64 targets

GCE guests seem to be using one loopback-mount more now which
+3 for CI exceeds available 8 loopback-devices.
Should be turned on again after we find how to get more
loopback-devices on GCE. Already a pity that we have to go
for emulation based on plain files on the LXD platforms.