x86/mm: Make x86_platform.guest.enc_status_change_*() return an error

x86/mm: Make x86_platform.guest.enc_status_change_*() return an error

TDX is going to have more than one reason to fail enc_status_change_prepare().

Change the callback to return errno instead of assuming -EIO. Change
enc_status_change_finish() too to keep the interface symmetric.

Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Reviewed-by: Dave Hansen <dave.hansen@intel.com>
Reviewed-by: Kai Huang <kai.huang@intel.com>
Reviewed-by: Michael Kelley <mhklinux@outlook.com>
Tested-by: Tao Liu <ltao@redhat.com>
Link: https://lore.kernel.org/r/20240614095904.1345461-8-kirill.shutemov@linux.intel.com