tools/libxc: Fix restoration of PV MSRs after migrate

tools/libxc: Fix restoration of PV MSRs after migrate

There are two bugs in process_vcpu_msrs() which clearly demonstrate that I
didn't test this bit of Migration v2 very well when writing it...

vcpu->msrsz is always expected to be a multiple of xen_domctl_vcpu_msr_t
records in a spec-compliant stream, so the modulo yields 0 for the msr_count,
rather than the actual number sent in the stream.

Passing 0 for the msr_count causes the hypercall to exit early, and hides the
fact that the guest handle is inserted into the wrong field in the domctl
union.

The reason that these bugs have gone unnoticed for so long is that the only
MSRs passed like this for PV guests are the AMD DBGEXT MSRs, which only exist
in fairly modern hardware, and whose use doesn't appear to be implemented in
any contemporary PV guests.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Wei Liu <wei.liu2@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
Release-acked-by: Julien Grall <julien.grall@linaro.org>