qemu_driver: check caps after starting block job

qemu_driver: check caps after starting block job

Currently we check qemuCaps before starting the block job. But qemuCaps
isn't available on a stopped domain, which means we get a misleading
error message in this case:

  # virsh domstate example
  shut off

  # virsh blockjob example vda
  error: unsupported configuration: block jobs not supported with this QEMU binary

Move the qemuCaps check into the block job so that we are guaranteed the
domain is running.

Signed-off-by: Michael Chapman <mike@very.puzzling.org>