domain: remove 'guest_type' field (and enum guest_type)

domain: remove 'guest_type' field (and enum guest_type)

The enum guest_type was introduced in commit 6c6492780ea "pvh prep:
introduce pv guest type and has_hvm_container macros" to allow a new guest
type, distinct from either PV or HVM guest types, to be added in commit
8271d6522c6 "pvh: introduce PVH guest type". Subsequently, commit
33e5c32559e "x86: remove PVHv1 code" removed this third guest type.

This patch removes the struct domain field and enumeration as the guest
type can now be trivially determined from the 'options' field.

Signed-off-by: Paul Durrant <paul.durrant@citrix.com>
Reviewed-by: "Roger Pau Monné" <roger.pau@citrix.com>
Acked-by: George Dunlap <George.Dunlap@eu.citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>