libxc/restore: Fix REC_TYPE_X86_PV_VCPU_XSAVE data auditing (take 2)

libxc/restore: Fix REC_TYPE_X86_PV_VCPU_XSAVE data auditing (take 2)

It turns out that a bug (since forever) in Xen causes XSAVE records to have
non-architectural behaviour on xsave-capable hardware, when a PV guest has not
touched the state.

In such a case, the data record returned from Xen is 2*uint64_t, both claiming
the (illegitimate) state of %xcr0 and %xcr0_accum being 0.

Adjust the bound in handle_x86_pv_vcpu_blob() to cope with this.

Fixes: 2a62c22715b "libxc/restore: Fix data auditing in handle_x86_pv_vcpu_blob()"
Reported-by: Igor Druzhinin <igor.druzhinin@citrix.com>
Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: Wei Liu <wl@xen.org>
(cherry picked from commit 0729830cc425a8ff27a3137e87b93768ae3c853c)
(cherry picked from commit d2aecd86c4481291b260869c47cf0a9a02321564)
(cherry picked from commit e43fc14ec58329813af876ed3b30899a04d65a08)