qemu: add support for libiscsi

qemu: add support for libiscsi

libiscsi provides a userspace iSCSI initiator.

The main advantage over the kernel initiator is that it is very
easy to provide different initiator names for VMs on the same host.
Thus libiscsi supports usage of persistent reservations in the VM,
which otherwise would only be possible with NPIV.

libiscsi uses "iscsi" as the scheme, not "iscsi+tcp".  We can change
this in the tests (while remaining backwards-compatible manner, because
QEMU uses TCP as the default transport for both Gluster and NBD).

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>