x86/sev: Relocate SNP guest messaging routines to common code

At present, the SEV guest driver exclusively handles SNP guest messaging. All
routines for sending guest messages are embedded within it.

To support Secure TSC, SEV-SNP guests must communicate with the AMD Security
Processor during early boot. However, these guest messaging functions are not
accessible during early boot since they are currently part of the guest
driver.

Hence, relocate the core SNP guest messaging functions to SEV common code and
provide an API for sending SNP guest messages.

No functional change, but just an export symbol added for
snp_send_guest_request() and dropped the export symbol on
snp_issue_guest_request() and made it static.

Signed-off-by: Nikunj A Dadhania <nikunj@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/20250106124633.1418972-5-nikunj@amd.com