Sets the hypervisor uri in launch_avd

In stable OS versions libvirt connects to qemu:///system by default,
but in Debian testing the default is qemy:///session. This change
allows the selection of the uri in the cmd line and defaults to system
if it's not provided.

Test: run locally
Change-Id: I60d598439d5d8d0d64f3dbaa96bde773b61e5a95