KVM: SVM: Use unsigned integers when dealing with ASIDs

Convert all local ASID variables and parameters throughout the SEV code
from signed integers to unsigned integers.  As ASIDs are fundamentally
unsigned values, and the global min/max variables are appropriately
unsigned integers, too.

Functionally, this is a glorified nop as KVM guarantees min_sev_asid is
non-zero, and no CPU supports -1u as the _only_ asid, i.e. the signed vs.
unsigned goof won't cause problems in practice.

Opportunistically use sev_get_asid() in sev_flush_encrypted_page() instead
of open coding an equivalent.

Reviewed-by: Tom Lendacky <thomas.lendacky@amd.com>
Link: https://lore.kernel.org/r/20240131235609.4161407-3-seanjc@google.com


Signed-off-by: Sean Christopherson <seanjc@google.com>