virttest.qemu_vm : add function to get vhost threads

This patch add a function to get vhost threads

Signed-off-by: Yunping Zheng <yunzheng@redhat.com>

Conflicts:
	virttest/qemu_vm.py
Merge pull request #633 from humanux/vhost_threads

virttest.qemu_vm : add function to get vhost threads
shared.deps: Adding build_install.py script to download git repo, build and install

In order to build git repos in the guest, ship this simple script.
It'll be used to build SPICE components, though it could be used
for other purposes.

Signed-off-by: Swapna Krishnan <skrishna@redhat.com>