VirtualDomain: avoid running "virsh uri" if hypervisor is set

Recent efforts to make this RA independent of a temporarily stopped
or blocking libvirtd missed the unconditional call to "virsh uri"
use to determine the hypervisor default. Since
OCF_RESKEY_hypervisor_default is actually not used anymore, we can
invoke virsh uri directly when setting OCF_RESKEY_hypervisor, but
only if the latter isn't already set.

Thus, users can avoid the superfluous "virsh uri" call altogether
by explicitly setting the hypervisor RA parameter.