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

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.
CA-110453: Unit tests

This patch contains the unit tests for CA-110453 workaround.