target/i386: SEV: Add support for setting TSC frequency for Secure TSC

Add support for configuring the TSC frequency when Secure TSC is enabled
in SEV-SNP guests through a new "tsc-frequency" property on SEV-SNP
guest objects, similar to the vCPU-specific property used by regular
guests and TDX. A new property is needed since SEV-SNP guests require
the TSC frequency to be specified during early SNP_LAUNCH_START command
before any vCPUs are created.

The user-provided TSC frequency is set through KVM_SET_TSC_KHZ before
issuing KVM_SEV_SNP_LAUNCH_START.

Attempts to set TSC frequency on both the SEV_SNP object and the cpu
object result in an error from KVM (on the vCPU ioctl), so do not add
separate checks for the same.

Sample command-line:
  -machine q35,confidential-guest-support=sev0 \
  -object sev-snp-guest,id=sev0,cbitpos=51,reduced-phys-bits=1,secure-tsc=on,tsc-frequency=2500000000

Co-developed-by: Ketan Chaturvedi <Ketan.Chaturvedi@amd.com>
Signed-off-by: Ketan Chaturvedi <Ketan.Chaturvedi@amd.com>
Co-developed-by: Nikunj A Dadhania <nikunj@amd.com>
Signed-off-by: Nikunj A Dadhania <nikunj@amd.com>
Signed-off-by: Naveen N Rao (AMD) <naveen@kernel.org>
Link: https://lore.kernel.org/r/af688610978f213a456a7753e1d9fe7d3a51e80a.1779281646.git.naveen@kernel.org
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>