selftests/vsock: improve logging in vmtest.sh

selftests/vsock: improve logging in vmtest.sh

Improve usability of logging functions. Remove the test name prefix from
logging functions so that logging calls can be made deeper into the call
stack without passing down the test name or setting some global. Teach
log function to accept a LOG_PREFIX variable to avoid unnecessary
argument shifting.

Remove log_setup() and instead use log_host(). The host/guest prefixes
are useful to show whether a failure happened on the guest or host side,
but "setup" doesn't really give additional useful information. Since all
log_setup() calls happen on the host, lets just use log_host() instead.

Signed-off-by: Bobby Eshleman <bobbyeshleman@meta.com>
Reviewed-by: Stefano Garzarella <sgarzare@redhat.com>
Link: https://patch.msgid.link/20251108-vsock-selftests-fixes-and-improvements-v4-1-d5e8d6c87289@meta.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>