vt-d: Use bitmap to solve domain-id limitation issue.

vt-d: Use bitmap to solve domain-id limitation issue.

The Capability register reports the domain-id width supported by
hardware. For implementations supporting less than 16-bit domainids,
unused bits of domain identifier field(87:72) in Context entry are
treated as reserved by hardware. For example, for an implementation
supporting 4-bit domain-ids, bits 87:76 of this field are treated as
reserved. 16 is a small number, overflow is easy to happen. What's
more,
context-entries programmed with the same domain identifier must always
reference the same address translation structure (through the ASR
field). So Dom16 will conflict with Dom0, and device assignment fails.

This patch implements a domaid id bitmap to solve above issue.

Signed-off-by: Weidong Han <weidong.han@intel.com>
Signed-off-by: Keir Fraser <keir.fraser@citrix.com>