Add support for QEMU built-in iSCSI initiator

QEMU has built-in iSCSI initiator using libiscsi and Libvirt
can handle it as same as other network storages. This blueprint
adds iSCSI support to Nova's network volume driver
(nova.virt.libvirt.LibvirtNetVolumeDriver).

Change-Id: I916746753aff5a6a7a68673e01fcf1ee9ecce2e6
Merge "Add support for QEMU built-in iSCSI initiator"