virt: sevguest: Fix bool function returning negative value

The function enc_payload() is wrongly declared bool but returns an
integer value. Correct it.

  [ bp: Massage commit message. ]

Fixes: fce96cf0 ("virt: Add SEV-SNP guest driver")
Signed-off-by: Haowen Bai <baihaowen@meizu.com>
Signed-off-by: Borislav Petkov <bp@suse.de>
Link: https://lore.kernel.org/r/1649930657-10837-1-git-send-email-baihaowen@meizu.com