Rework how selfhosted tests interact with runners.

Rework how selfhosted tests interact with runners.

Previously there was one runner per test target (mostly VMs).  This had
a few limitations:
 - multiple tests that ran on the same target (eg multiple build
   configs) were serialized on availability or that runner.
 - it needed manual balancing of VMs over host machines.

To address this, make VMs that use ephemeral disks (ie most of them)
all use a pool of runners with the "libvirt" label.  This requires that
we distinguish between "host" and "target" for those.  Native runners
and VMs with persistent disks (eg the constantly-updated snapshot ones)
specify the same host and target.

This should improve test throughput.