security_util: Don't error on macOS when getting/setting/moving XATTRs

security_util: Don't error on macOS when getting/setting/moving XATTRs

There are three internal APIs implemented in this security_util
file: virSecurityGetRememberedLabel(),
virSecuritySetRememberedLabel() and
virSecurityMoveRememberedLabel() for getting, setting and moving
remembered seclabel. All three have a special return value of -2
when XATTRs are not supported (for whatever reason) and callers
are expected to handle it gracefully. However, after my commit of
v5.7.0-rc1~115 it may happen that one of the three functions
returned -1 even though XATTRs are not supported (and thus -2
should have been returned).

Fixes: 7cfb7aab573a031880a1f4fd20747843fea109ba
Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Roman Bolshakov <r.bolshakov@yadro.com>
Reviewed-by: Andrea Bolognani <abologna@redhat.com>