Refactor get_iscsi_initiator to a common location

Refactor get_iscsi_initiator to a common location so that other
drivers can use it, without reaching into virt/libvirt. This patch
updates the current baremetal volume driver to the new shared location.

Change-Id: I30539f4bb917e486739f797bb6a9579e1cdffd22
Merge "Refactor get_iscsi_initiator to a common location"