Fix run.py when build/ fs does not support direct IO

When the build/ directory is on tmpfs then 'cache=none' option makes
qemu fail with error:

    could not open disk image build/release/usr.img : Invalid argument

Using tmpfs for build/ makes a _huge_ difference in build time on some
setups.

This patch adds fallback to 'unsafe' mode when direct IO is not
supported.

Signed-off-by: Tomasz Grabiec <tgrabiec@cloudius-systems.com>