target/i386: Make invtsc migratable when user sets tsc-khz explicitly

target/i386: Make invtsc migratable when user sets tsc-khz explicitly

When user sets tsc-frequency explicitly, the invtsc feature is actually
migratable because the tsc-frequency is supposed to be fixed during the
migration.

See commit d99569d9d856 ("kvm: Allow invtsc migration if tsc-khz
is set explicitly") for referrence.

Signed-off-by: Xiaoyao Li <xiaoyao.li@intel.com>
Link: https://lore.kernel.org/r/20240814075431.339209-10-xiaoyao.li@intel.com
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>