qemu: hotplug: Don't format NULL in %s in qemuHotplugPrepareDiskAccess
qemu: hotplug: Don't format NULL in %s in qemuHotplugPrepareDiskAccess

The warning messages which include the disk source could potentially
format NULL using %s as virDomainDiskGetSource may return NULL for e.g.
NBD disks. As most of the APIs are NOOP for remote disks the usage of
the source string only should be fine for now.

Signed-off-by: Peter Krempa <2cf5c04c61aa466e4a47bfedc747d17279c72ffc@redhat.com>
Reviewed-by: Ján Tomko <4cab11cfb98d3c937327354a78eb07dbb6ee2bc6@redhat.com>

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