qemu: hotplug: Add helper to initialize/teardown new disks for VMs

qemu: hotplug: Add helper to initialize/teardown new disks for VMs

When we are changing media (or doing other hotplug operations) we need
to setup cgroups, locking and seclabels on the new disk. This is a
multi-step process where every piece can fail. To simplify dealing with
this introduce qemuDomainPrepareDisk that similarly to
qemuDomainPrepareDiskChainElement initializes/tears down  a whole new
disk to be used with the domain.

Additionally the function supports passing a different source struct for
media changes of cdroms that will be refactored later.