glib: add compatibility interface for g_get_monotonic_time()

glib: add compatibility interface for g_get_monotonic_time()

This patch fixes compilation errors when building against glib <2.28.0
due to the missing g_get_monotonic_time() function.

The compilation error in tests/libqos/virtio.c was introduced in commit
70556264a89a268efba1d7e8e341adcdd7881eb4 ("libqos: use microseconds
instead of iterations for virtio timeout").

Add a simple g_get_monotonic_time() implementation to glib-compat.h
based on code from vhost-user-test.c.

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
[Igor: add G_TIME_SPAN_SECOND, include glib-compat.h in libqtest.h]
Signed-off-by: Igor Mammedov <imammedo@redhat.com>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>