Fix buffer overrun when suspending VMs with many vCPUs

Fix buffer overrun when suspending VMs with many vCPUs

Dynamically allocate the KDPC array. __Section is defined as a PAGE_SIZE
region, which can only contain a limited number of KDPC objects in addition
to the SYNC_CONTEXT header. Dynamic allocation of the KDPC objects will
remove this restriction.

Signed-off-by: Owen Smith <owen.smith@cloud.com>
Always dynamically allocate KDPC objects, allowing us to get rid of
__Section and define SyncContext as a simple static global. Hence amend
the original commit comment.

Also use __AllocatePoolWithTag() and __FreePoolWithTag() rather than
open-coding them.

Signed-off-by: Paul Durrant <paul@xen.org>