virt: sev-guest: Choose the VMPCK key based on executing VMPL

virt: sev-guest: Choose the VMPCK key based on executing VMPL

Currently, the sev-guest driver uses the vmpck-0 key by default. When an
SVSM is present, the kernel is running at a VMPL other than 0 and the
vmpck-0 key is no longer available. If a specific vmpck key has not be
requested by the user via the vmpck_id module parameter, choose the
vmpck key based on the active VMPL level.

Signed-off-by: Tom Lendacky <thomas.lendacky@amd.com>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Link: https://lore.kernel.org/r/b88081c5d88263176849df8ea93e90a404619cab.1717600736.git.thomas.lendacky@amd.com