x86/sev: Provide guest VMPL level to userspace

x86/sev: Provide guest VMPL level to userspace

Requesting an attestation report from userspace involves providing the VMPL
level for the report. Currently any value from 0-3 is valid because Linux
enforces running at VMPL0.

When an SVSM is present, though, Linux will not be running at VMPL0 and only
VMPL values starting at the VMPL level Linux is running at to 3 are valid. In
order to allow userspace to determine the minimum VMPL value that can be
supplied to an attestation report, create a sysfs entry that can be used to
retrieve the current VMPL level of the kernel.

  [ bp: Add CONFIG_SYSFS ifdeffery. ]

Signed-off-by: Tom Lendacky <thomas.lendacky@amd.com>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Link: https://lore.kernel.org/r/fff846da0d8d561f9fdaf297dcf8cd907545a25b.1717600736.git.thomas.lendacky@amd.com