install-autotest-server.sh - checkout master branches on client/server tests

Added a call to checkout master branch for both tests subrepos,
this way they will represent latest upstream.

Changes from v1:
 * Also update virt-tests, which is a submodule of the client
   tests submodule

Signed-off-by: Julius Gawlas <julius.gawlas@hp.com>
Signed-off-by: Lucas Meneghel Rodrigues <lmr@redhat.com>