cutils: refine strtol error handling in parse_debug_env

Avoid truncation of a 64-bit long to a 32-bit int, and check for errno
(especially ERANGE).

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>
