qemu: Introduce qemuDomainPrepareDiskSource

qemu: Introduce qemuDomainPrepareDiskSource

Introduce a function to setup any TLS needs for a disk source.

If there's a configuration or other error setting up the disk source
for TLS, then cause the domain startup to fail.

For VxHS, follow the chardevTLS model where if the src->haveTLS hasn't
been configured, then take the system/global cfg->haveTLS setting for
the storage source *and* mark that we've done so via the tlsFromConfig
setting in storage source.

Next, if we are using TLS, then generate an alias into a virStorageSource
'tlsAlias' field that will be used to create the TLS object and added to
the disk object in order to link the two together for QEMU.

Signed-off-by: John Ferlan <jferlan@redhat.com>