[HVM] Add a concept of HVM parameters to the hypervisor.

[HVM] Add a concept of HVM parameters to the hypervisor.

Each HVM domain has a space of HVM parameters associated with it,
and these can be manipulated via a new hvm_op hypercall. This means
that the hypervisor no longer needs to parse the hvm_info table, so
remove that code.

Signed-off-by: Steven Smith <ssmith@xensource.com>
Signed-off-by: Keir Fraser <keir@xensource.com>