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.
CP-5736: Unit tests