virt: sev-guest: Carve out SNP message context structure

Currently, the sev-guest driver is the only user of SNP guest messaging.
The snp_guest_dev structure holds all the allocated buffers, secrets page
and VMPCK details. In preparation for adding messaging allocation and
initialization APIs, decouple snp_guest_dev from messaging-related
information by carving out the guest message context
structure(snp_msg_desc).

Incorporate this newly added context into snp_send_guest_request() and all
related functions, replacing the use of the snp_guest_dev.

No functional change.

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/20241009092850.197575-7-nikunj@amd.com