qemuDomainPrepareHostdev: Don't base backend nodename on device alias

qemuDomainPrepareHostdev: Don't base backend nodename on device alias

QEMU's blockdev nodenames which are used to back SCSI/iSCSI hostdevs are
limited to 32 characters. If a user passes a very long user alias as
name of the host device it's easy to end up with a too-long nodename.

To prevent this from happening don't base the nodename on the possibly
user-specified alias but on the normal sequential node name generator.

We then store the name in the status XML for further use.

Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>