Allow --local-system-image to be file

This commit removes the restriction that --local-system-image must be
followed by a directory containing "system.img". The argument can be a
file, which is flexible for the users and the infrastructures.

Test: acloud-dev create --local-instance --local-image \
      --local-system-image ./system.img
Test: acloud-dev create --local-instance --local-image \
      --local-system-image ./system.img --avd-type goldfish
Bug: 185094559
Change-Id: I523e8c87050cd34d34bb7171176992c8ebb5d432
9 files changed