fix(image_to_vm): Mount source rootfs as ro when creating cpio.
fix(image_to_vm): Mount source rootfs as ro when creating cpio.

When building with a write-protected root mounting as rw, the default,
fails.

sigma/coreos-scripts,glevand/coreos--scripts,mjg59/scripts,zhang0137/scripts,endocode/scripts,coreos/scripts,marineam/coreos-scripts,zhang0137/scripts,mjg59/scripts,nicescale/scripts,bcwaldon/coreos-scripts,smilart/scripts,BugRoger/scripts,BugRoger/scripts,crawford/scripts,nicescale/scripts,gtank/scripts,glevand/coreos--scripts,mischief/scripts,nicescale/scripts,marineam/coreos-scripts,trnubo/scripts,fivethreeo/scripts,crawford/scripts,sigma/coreos-scripts,andrejro/coreos--scripts,BugRoger/scripts,fivethreeo/scripts,gtank/scripts,bcwaldon/coreos-scripts,bcwaldon/coreos-scripts,mischief/scripts,fivethreeo/scripts,marineam/coreos-scripts,mischief/scripts,mjg59/scripts,zhang0137/scripts,gtank/scripts,trnubo/scripts,andrejro/coreos--scripts,andrejro/coreos--scripts,smilart/scripts,trnubo/scripts,smilart/scripts,coreos/scripts,sigma/coreos-scripts,vmware/coreos-scripts,endocode/scripts,vmware/coreos-scripts,crawford/scripts,endocode/scripts,vmware/coreos-scripts