This patch modifies the blkback driver to perform a security check 
(via an ACM get_decision hypercall) before connecting a vbd to a 
domain.  While the security checks performed in the xm tools are 
convenient for the users, the check added here is intended to 
strengthen the security guarantees.

Note that the security check in blkback is only enabled when 
ACM_SECURITY=y in Config.mk.

Signed-off-by: Bryan D. Payne <bdpayne@us.ibm.com>
Signed-off-by: Reiner Sailer <sailer@us.ibm.com>