Improve error reporting when libvirtd is not installed

Improve error reporting when libvirtd is not installed

Currently if you try to connect to a local libvirtd when
libvirtd is not in $PATH, you'll get an error

  error: internal error invalid use of command API

This is because remoteFindDaemonPath() returns NULL, which
causes us to pass NULL into virNetSocketConnectUNIX which
in turn causes us to pass NULL into virCommandNewArgList.

Adding missing error checks improves this to

  error: internal error Unable to locate libvirtd daemon in $PATH

* src/remote/remote_driver.c: Report error if libvirtd
  cannot be found
* src/rpc/virnetsocket.c: Report error if caller requested
  spawning of daemon, but provided no binary path