KVM: SVM: Enable Secure TSC for SNP guests

KVM: SVM: Enable Secure TSC for SNP guests

Add support for Secure TSC, allowing userspace to configure the Secure TSC
feature for SNP guests. Use the SNP specification's desired TSC frequency
parameter during the SNP_LAUNCH_START command to set the mean TSC
frequency in KHz for Secure TSC enabled guests.

Always use kvm->arch.arch.default_tsc_khz as the TSC frequency that is
passed to SNP guests in the SNP_LAUNCH_START command.  The default value
is the host TSC frequency.  The userspace can optionally change the TSC
frequency via the KVM_SET_TSC_KHZ ioctl before calling the
SNP_LAUNCH_START ioctl.

Introduce the read-only MSR GUEST_TSC_FREQ (0xc0010134) that returns
guest's effective frequency in MHZ when Secure TSC is enabled for SNP
guests. Disable interception of this MSR when Secure TSC is enabled. Note
that GUEST_TSC_FREQ MSR is accessible only to the guest and not from the
hypervisor context.

Co-developed-by: Ketan Chaturvedi <Ketan.Chaturvedi@amd.com>
Signed-off-by: Ketan Chaturvedi <Ketan.Chaturvedi@amd.com>
Reviewed-by: Kai Huang <kai.huang@intel.com>
Reviewed-by: Tom Lendacky <thomas.lendacky@amd.com>
Signed-off-by: Nikunj A Dadhania <nikunj@amd.com>
[sean: contain Secure TSC to sev.c]
Link: https://lore.kernel.org/r/20250819234833.3080255-9-seanjc@google.com
[sean: return -EINVAL if TSC frequency is '0']
Signed-off-by: Sean Christopherson <seanjc@google.com>