kvmclock: fix clock_is_reliable on migration from QEMU < 2.9

kvmclock: fix clock_is_reliable on migration from QEMU < 2.9

When migrating from a pre-2.9 QEMU, no clock_is_reliable flag is
transferred. We should assume that the source host has an unreliable
KVM_GET_CLOCK, rather than using whatever was determined locally, to
ensure that any drift from the TSC-based value calculated by the guest
is corrected.

Signed-off-by: Michael Chapman <mike@very.puzzling.org>
Message-Id: <20180406053406.774-1-mike@very.puzzling.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>