nixos/libvirtd-service: fix for garbage collected emulator paths

libvirtd puts the full path of the emulator binary in the machine config
file. But this path can unfortunately be garbage collected while still
being used by the virtual machine. Then this happens:

Error starting domain: Cannot check QEMU binary /nix/store/z5c2xzk9x0pj6x511w0w4gy9xl5wljxy-qemu-1.5.2-x86-only/bin/qemu-kvm: No such file or directory

Fix by updating the emulator path on each service startup to something
valid (re-scan $PATH).