qemu_process: Document qemuProcessPrepare{Domain,Host}() order

qemu_process: Document qemuProcessPrepare{Domain,Host}() order

The domain startup process is split into multiple phases. One of
them is preparing the domain (at that point live) XML, private
data, various paths, etc - see qemuProcessPrepareDomain(); the
other prepares the host - see qemuProcessPrepareHost(). It's
obvious that the domain XML preparation function must be called
before the host preparation function (e.g. the host preparation
might try to create a file which path is generated in the domain
preparation phase). Nevertheless, let's document this
expectation.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>