sheepdog: support iSCSI multipath

This patch implements iSCSI multipath support in sheepdog backing
store driver. Below is a brief description of the feature:

The main problem of enabling iSCSI multipath for sheepdog's VDI is
keeping coherence of tgtd's internal inode data structure. Assume tgt
A and B have logical units which point same sheepdog VDI. After
initialization of the logical units, A and B have copies of inode
object in their address space (let's X denote this object). When an
initiator sends write request to A and the request causes COW, A
creates a new object and updates the inode object X (let's X' denote
the updated one).

After the update by A, A can die with hardware faults. In such a case,
the initiator detects a connection error and switch its target to B if
iSCSI multipath is configured correctly. But the problem can arise at
this point. Because B doesn't know the update caused by A and its
internal inode object is still X, not X'. B creates requests based on
X, so it can read obsolete data which is already hidden by X' or
update X' in wrong manner.

For solving the above problem, sheepdog implements inode object
coherence protocol which mocks MSI protocol of shared memory multicore
processors. With this mechanism sheepdog can prevent requests which
are based on obsolete inode objects. This patch let sheepdog driver of
tgtd adopt the coherence protocol.

Signed-off-by: Hitoshi Mitake <mitake.hitoshi@lab.ntt.co.jp>