Only enforce check for YAJL when starting a VM

Only enforce check for YAJL when starting a VM

The previous check for YAJL would have many undesirable
consequences, the most important being that it caused the
capabilities XML to lose all <guest> elements. There is
no user visible feedback as to what is wrong in this respect,
merely a syslog message. The empty capabilities causes
libvirtd to then throw away all guest XML configs that are
stored.

This changes the code so that the check for YAJL is only
performed at the time we attempt to spawn a QEMU process

error: Failed to start domain vm-vnc
error: unsupported configuration: this qemu binary requires libvirt to be compiled with yajl

Signed-off-by: Daniel P. Berrange <berrange@redhat.com>