Optimize Qemu installation to speed up vhost tests

Currently Qemu is being installed if there is a change of qsz parameter
between tests/suites. Qemu is installed always into the same directory. This
patch changes the default behavior to install qemu over and install
pathced version to separate directory. It also disables force install.

Change-Id: I0d7493a02b026a6ae4a5ea8bacf54656de9db567
Signed-off-by: Peter Mikus <pmikus@cisco.com>