Add method to create empty writable partition.

For now this is just a sparse file to be used as raw image, but in
future it will be a QCOW2 image.

Bug: 190503448
Test: vm create-partition /data/local/tmp/blah 1000000
Change-Id: Ic07cd96b1a05bfaade1fa48d75d1ed3167d32795
3 files changed