hyperv: make SynIC version msr constant

The value of HV_X64_MSR_SVERSION is initialized once at vcpu init, and
is reset to zero on vcpu reset, which is wrong.

It is supposed to be a constant, so drop the field from X86CPU, set the
msr with the constant value, and don't bother getting it.

Signed-off-by: Roman Kagan <rkagan@virtuozzo.com>
Message-Id: <20171122181418.14180-4-rkagan@virtuozzo.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
