CP-5736: ISCSISR - force_tapdisk

This is a workaround to make it possible to use a domU provided iSCSI
target as a raw iSCSI SR. By using the force_tapdisk parameter during SR
creation, disks will be accessed through tapdisk aio which copies
blocks, preventing page sharing issues between guests. This is a
workaround for the bug CA-110453.

(cherry picked from commit 8da7ba62bd92bce057ef5b9ce5919a40a2b5b42e)

Reviewed-by: Germano Percossi <germano.percossi@citrix.com>
Signed-off-by: Keith Petley <keith.petley@citrix.com>
Imported-by: Mate Lakat <mate.lakat@citrix.com>
CP-5736: Unit tests

Unit tests and scripts to setup the environment. See tests/README for
details.

(cherry picked from commit de8a7cf26aa571d2f39a13b472e0539215a3cd45)

Reviewed-by: Germano Percossi <germano.percossi@citrix.com>
Signed-off-by: Keith Petley <keith.petley@citrix.com>
Imported-by: Mate Lakat <mate.lakat@citrix.com>