x86/sev: Cleanup vc_handle_msr()

Carve out the MSR_SVSM_CAA into a helper with the suggestion that
upcoming future users should do the same. Rename that silly exit_info_1
into what it actually means in this function - whether the MSR access is
a read or a write.

No functional changes.

Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Reviewed-by: Tom Lendacky <thomas.lendacky@amd.com>
Reviewed-by: Pankaj Gupta <pankaj.gupta@amd.com>
Link: https://lore.kernel.org/r/20241106172647.GAZyum1zngPDwyD2IJ@fat_crate.local