virsh: change vshCommandOptString return type and fix const-correctness
virsh: change vshCommandOptString return type and fix const-correctness

This function should return pointer to const, because we don't want
to change command option value. Therefore we can ensure const-correctness.
(cherry picked from commit dc3d60ecd594b33ffc6956cf2ff37f87825e7849)

Signed-off-by: Steve Hodgson <eb20a6615749cf04ff61aa37e18f749ed6d01446@solarflare.com>

sshah-solarflare/Libvirt-PCI-passthrough-,sshah-solarflare/Libvirt-PCI-passthrough-,sshah-solarflare/Libvirt-PCI-passthrough-,sshah-solarflare/Libvirt-PCI-passthrough-,sshah-solarflare/Libvirt-PCI-passthrough-