tools: Delete XEN_DOMCTL_disable_migrate

tools: Delete XEN_DOMCTL_disable_migrate

It is conceptually wrong for this information to exist in the hypervisor in
the first place.  Only the toolstack is capable of correctly reasoning about
the non-migrateability of guests.

This hypercall has only ever existed to control the visibility of the
Invariant TSC flag to the guest.  Now that we have properly disentanged that
and moved ITSC into the guests CPUID policy, delete this hypercall.

Furthermore, this fixes a corner case where Xen would override the toolstacks
choice of ITSC for a xenstore stubdomain.  That said, init-xenstore-domain
doesn't currently ask for ITSC, and MiniOS doesn't know what ITSC is either,
so nothing actually changes in practice.

Bump the DOMCTL_INTERFACE_VERSION for 4.15

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>