x86/domctl: Fix TOCTOU race with the use of XEN_DOMCTL_getvcpuextstate

x86/domctl: Fix TOCTOU race with the use of XEN_DOMCTL_getvcpuextstate

A toolstack must call XEN_DOMCTL_getvcpuextstate twice; first to find the size
of the buffer to use, and a second time to get the actual content.

The reported size was based on v->arch.xcr0_accum, but a guest which extends
its xcr0_accum between the two hypercalls will cause the toolstack to fail the
evc->size != size check, as the provided buffer is now too small.  This causes
a hard error during the final phase of migration.

Instead, return a size based on xfeature_mask, which is the maximum size Xen
will ever permit.  The hypercall must now tolerate a toolstack-provided buffer
which is overly large (for the case where a guest isn't using all available
xsave states), and should write back how much data was actually written into
the buffer.

As the query for size now has no dependence on vcpu state, the vcpu_pause()
can be omitted for a small performance improvement.

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