qtest: replace gettimeofday with GTimer

glib provides a convenience helper to measure elapsed time. It isn't
subject to wall-clock time changes.

Note that this changes the initial OPENED time, which used to print the
current time.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Laurent Vivier <laurent@vivier.eu>
Message-Id: <20220307070401.171986-3-marcandre.lureau@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>