virt: sev-guest: Consolidate SNP guest messaging parameters to a struct

Add a snp_guest_req structure to eliminate the need to pass a long list of
parameters. This structure will be used to call the SNP Guest message
request API, simplifying the function arguments.

Update the snp_issue_guest_request() prototype to include the new guest
request 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/20241009092850.197575-5-nikunj@amd.com