x86/hvm: add stricter permissions checks to ioreq server control plane

x86/hvm: add stricter permissions checks to ioreq server control plane

There has always been an intention in the ioreq server API that only the
domain that creates an ioreq server should be able to manipulate it.
However, so far, nothing has enforced this. This means that two domains
with DM_PRIV over a target domain can currently manipulate each others
ioreq servers.

A previous patch added code to take a reference and store a pointer to the
domain that creates an ioreq server. This patch now adds checks to the
functions that manipulate the ioreq server to make sure they are being
called by the same domain.

Signed-off-by: Paul Durrant <paul.durrant@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>