x86/sev: Carve out and export SNP guest messaging init routines

Currently, the sev-guest driver is the only user of SNP guest messaging. All
routines for initializing SNP guest messaging are implemented within the
sev-guest driver and are not available during early boot.

In preparation for adding Secure TSC guest support, carve out APIs to allocate
and initialize the guest messaging descriptor context and make it part of
coco/sev/core.c. As there is no user of sev_guest_platform_data anymore,
remove the structure.

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/20250106124633.1418972-4-nikunj@amd.com