Update image_to_vm to run sync again prior to detaching loop device.
Update image_to_vm to run sync again prior to detaching loop device.

Prevents errors like this:
  loop: can't delete device /dev/loop0: Device or resource busy

See http://build.chromium.org/p/chromiumos/builders/x86%20pineview%20full/builds/119/steps/BuildTarget/logs/stdio

BUG=chromium-os:19150
TEST=Ran image_to_vm.sh

Change-Id: I8300b4770e1b194045fc068b08ff549984361897
Reviewed-on: http://gerrit.chromium.org/gerrit/6024
Tested-by: David James <f231dc574bbeb62604660cbc162d1b8642a0a3af@chromium.org>
Reviewed-by: David James <f231dc574bbeb62604660cbc162d1b8642a0a3af@chromium.org>

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