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If host root filesystem is writable and guest system image is
EXT format, then the guest root filesystem should also be writable and
StartArcVmRequest.rootfs_writable is set to true.

This CL is a part of the change for b/144154973.

Bug: 144154973
TEST=1. On host, 'mount -o remount,rw /'
     2. Make sure guest system image is EXT4
     3. On host, 'restart ui'
     4. On guest, 'mount -o remount,rw /'
     5. Guest root filesystem should be writable

Change-Id: Id58ab406ca74f240aeadcfef2888c6578a6c5857
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1916523
Commit-Queue: Cherie Cheung <cherieccy@chromium.org>
Reviewed-by: Yusuke Sato <yusukes@chromium.org>
Cr-Commit-Position: refs/heads/master@{#716097}