x86/hvm: Don't corrupt the HVM context stream when writing the MSR record

x86/hvm: Don't corrupt the HVM context stream when writing the MSR record

Ever since it was introduced in c/s bd1f0b45ff, hvm_save_cpu_msrs() has had a
bug whereby it corrupts the HVM context stream if some, but fewer than the
maximum number of MSRs are written.

_hvm_init_entry() creates an hvm_save_descriptor with length for
msr_count_max, but in the case that we write fewer than max, h->cur only moves
forward by the amount of space used, causing the subsequent
hvm_save_descriptor to be written within the bounds of the previous one.

To resolve this, reduce the length reported by the descriptor to match the
actual number of bytes used.

A typical failure on the destination side looks like:

    (XEN) HVM4 restore: CPU_MSR 0
    (XEN) HVM4.0 restore: not enough data left to read 56 MSR bytes
    (XEN) HVM4 restore: failed to load entry 20/0

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>