i386/tdx: Set APIC bus rate to match with what TDX module enforces

i386/tdx: Set APIC bus rate to match with what TDX module enforces

TDX advertises core crystal clock with cpuid[0x15] as 25MHz for TD
guests and it's unchangeable from VMM. As a result, TDX guest reads
the APIC timer at the same frequency, 25MHz.

While KVM's default emulated frequency for APIC bus is 1GHz, set the
APIC bus rate to match with TDX explicitly to ensure KVM provide correct
emulated APIC timer for TD guest.

Signed-off-by: Xiaoyao Li <xiaoyao.li@intel.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Reviewed-by: Zhao Liu <zhao1.liu@intel.com>
Link: https://lore.kernel.org/r/20250508150002.689633-15-xiaoyao.li@intel.com
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>