qemu: domain: Tolerate NULL @disk in qemuDomainPrepareDiskSourceData
qemu: domain: Tolerate NULL @disk in qemuDomainPrepareDiskSourceData

In some cases we want to prepare a @src which is not meant to belong to
a disk and thus does not require us to copy the data. Allow passing in
NULL @disk into qemuDomainPrepareDiskSourceData.

Signed-off-by: Peter Krempa <2cf5c04c61aa466e4a47bfedc747d17279c72ffc@redhat.com>
Reviewed-by: Daniel Henrique Barboza <627b089ad62deae9aaf06ccfa2c9056df28927e0@gmail.com>

crobinso/libvirt,crobinso/libvirt,nertpinx/libvirt,fabianfreyer/libvirt,fabianfreyer/libvirt,libvirt/libvirt,fabianfreyer/libvirt,zippy2/libvirt,nertpinx/libvirt,jardasgit/libvirt,jardasgit/libvirt,zippy2/libvirt,crobinso/libvirt,crobinso/libvirt,libvirt/libvirt,jardasgit/libvirt,jardasgit/libvirt,olafhering/libvirt,jfehlig/libvirt,zippy2/libvirt,jfehlig/libvirt,fabianfreyer/libvirt,libvirt/libvirt,zippy2/libvirt,jfehlig/libvirt,jfehlig/libvirt,nertpinx/libvirt,jardasgit/libvirt,libvirt/libvirt,olafhering/libvirt,fabianfreyer/libvirt,nertpinx/libvirt,olafhering/libvirt,nertpinx/libvirt,olafhering/libvirt