x86/xsave: remove xfeat_mask checking from validate_xstate()

x86/xsave: remove xfeat_mask checking from validate_xstate()

validate_xsave() is called codepaths which load new vcpu xsave state from
XEN_DOMCTL_{setvcpuextstate,sethvmcontext}, usually as part of migration.  In
both cases, this is the xfeature_mask of the saving Xen rather than the
restoring Xen.

Given that the xsave state itself is checked for consistency and validity on
the current cpu, checking whether it was valid for the cpu before migration is
not interesting (or indeed relevant, as the error can't be distinguished from
the other validity checking).

This change removes the need to pass the saving Xen's xfeature_mask,
simplifying the toolstack code and migration stream format in this area.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>