Multiple tapdisk2 processes may use the same parent disk images (later used to

Multiple tapdisk2 processes may use the same parent disk images (later used to
detect sharable memory pages). This patch establishes unique id for each disk
image opened by tapdisk2, and stores it in shared memory region, thus making it
available to the remaining tapdisk2s.

Signed-off-by: Grzegorz Milos <Grzegorz.Milos@citrix.com>