qemu: Move qemuExtDevicesStop() before removing the pidfiles

A qemu guest which has virtiofs config fails to start if the previous
starting failed because of invalid option or something.

That's because the virtiofsd isn't killed by virPidFileForceCleanupPath()
on the former failure because the pidfile was already removed by
virFileDeleteTree(priv->libDir) in qemuProcessStop(), so
virPidFileForceCleanupPath() just returned.

Move qemuExtDevicesStop() before virFileDeleteTree(priv->libDir) so that
virPidFileForceCleanupPath() can kill virtiofsd correctly.

For example of the reproduction:

  # virsh start guest
  error: Failed to start domain guest
  error: internal error: process exited while connecting to monitor: qemu-system-x86_64: -foo: invalid option

  ... fix the option ...

  # virsh start guest
  error: Failed to start domain guest
  error: Cannot open log file: '/var/log/libvirt/qemu/guest-fs0-virtiofsd.log': Device or resource busy
  #

Signed-off-by: Masayoshi Mizuma <m.mizuma@jp.fujitsu.com>
Reviewed-by: Michal Privoznik <mprivozn@redhat.com>