x86/virt: Provide "nosnp" boot option for sev kernel command line

Provide a "nosnp" kernel command line option to prevent enabling of the RMP
and SEV-SNP features in the host/hypervisor. Not initializing the RMP
removes system overhead associated with RMP checks.

  [ bp: Actually make it a HV-only cmdline option. ]

Co-developed-by: Eric Van Tassell <Eric.VanTassell@amd.com>
Signed-off-by: Eric Van Tassell <Eric.VanTassell@amd.com>
Signed-off-by: Pavan Kumar Paluri <papaluri@amd.com>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Reviewed-by: Tom Lendacky <thomas.lendacky@amd.com>
Link: https://lore.kernel.org/r/20241014130948.1476946-3-papaluri@amd.com