x86/hvm/ioreq: maintain an array of ioreq servers rather than a list

x86/hvm/ioreq: maintain an array of ioreq servers rather than a list

A subsequent patch will remove the current implicit limitation on creation
of ioreq servers which is due to the allocation of gfns for the ioreq
structures and buffered ioreq ring.

It will therefore be necessary to introduce an explicit limit and, since
this limit should be small, it simplifies the code to maintain an array of
that size rather than using a list.

Also, by reserving an array slot for the default server and populating
array slots early in create, the need to pass an 'is_default' boolean
to sub-functions can be avoided.

Some function return values are changed by this patch: Specifically, in
the case where the id of the default ioreq server is passed in, -EOPNOTSUPP
is now returned rather than -ENOENT.

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