CA-147141: Fix includes in tapdisk-interface.h

As tapdisk-interface.h was referencing to types which were not defined,
so automatic mock creation failed. With this fix, the file should
compile on its own.

Signed-off-by: Mate Lakat <mate.lakat@citrix.com>
CA-147141: Move tapdisk_vbd_add_image to c file

tapdisk_vbd_add_image is a static inline method, and it's only client
was living in tapdisk-vbd.c, so moving it to there.

Signed-off-by: Mate Lakat <mate.lakat@citrix.com>
CA-147141: Move tapdisk_vbd_is_last_image to C file

This function was only used in tapdisk-vbd.c, so moving it there.

Signed-off-by: Mate Lakat <mate.lakat@citrix.com>
CA-147141: Move tapdisk_vbd_first_image to C file

Signed-off-by: Mate Lakat <mate.lakat@citrix.com>
CA-147141: Move tapdisk_vbd_last_image to C file

Signed-off-by: Mate Lakat <mate.lakat@citrix.com>
CA-147141: Move tapdisk_vbd_next_image to src file

Signed-off-by: Mate Lakat <mate.lakat@citrix.com>
CA-147141: Add example noop unit tests

Signed-off-by: Mate Lakat <mate.lakat@citrix.com>
CA-147141: Test tapdisk_xenblkif_complete_request

Signed-off-by: Mate Lakat <mate.lakat@citrix.com>