qemuBuildChardevCommand: Split creation of the command and setup of other objects

qemuBuildChardevCommand: Split creation of the command and setup of other objects

Completely seprate the creation of the commandline string from the setup
of other objects instantiated on the commandline.

'qemuBuildChardevCommand' will aggregate the setup of individual
parameters such as -add-fd and setup of TLS and the -chardev parameter
itself while the code formatting the commandline will be moved into
qemuBuildChardevStr.

'fdset' names are then stored in qemuDomainChrSourcePrivate.

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