qemuDomainAttachDiskGeneric: Move setup of disk into qemuDomainAttachDeviceDiskLiveIn...

qemuDomainAttachDiskGeneric: Move setup of disk into qemuDomainAttachDeviceDiskLiveInternal

qemuDomainAttachDeviceDiskLiveInternal already sets up certain pieces of
the disk definition so it's better suited to move the setup of the
virStorageSource structs, granting access to the storage and allocation
of the alias from qemuDomainAttachDiskGeneric which will be just
handling the qemu interaction.

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