CA-110453: raw iSCSI target provided by domU could not be used

On a XenServer with domU1 and domU2: domU1 is an iSCSI target, dom0 us
using that a a raw iSCSI SR. domU2 is accessing that SR and this
operation fails. This patch adds an additional force_tapdisk parameter
to force the use of tapdisk so that the mentioned configuration works.
refactor: rename
Simplify test by overriding load method
vdi_type is modified by force_tapdisk
Remove duplicated line
Amend commit according to Robert's comments
Fix bug: AttributeError object has no attribute force_tapdisk

As load() is called from __init__(), force_tapdisk was undefined at that point
use existing whitespace convention
Add prerequisite install script
Updated env setup
Updated prerequisites install
build.sh
virtualenv fix
compile snapwatchd
Added test runner
Make run_tests.sh executable
Document how to set up the environment and how to run the unittests
Remove sm prefix
Add .env directory to gitignore
Refactor - move test_ISCSISR.py to tests/