qemu: Introduce qemuDomainChardevPrivatePtr

qemu: Introduce qemuDomainChardevPrivatePtr

Modeled after the qemuDomainHostdevPrivatePtr (commit id '27726d8c'),
create a privateData pointer in the _virDomainChardevDef to allow storage
of private data for a hypervisor in order to at least temporarily store
secret data for usage during qemuBuildCommandLine.

NB: Since the qemu_parse_command (qemuParseCommandLine) code is not
expecting to restore the secret data, there's no need to add code
code to handle this new structure there.

Signed-off-by: John Ferlan <jferlan@redhat.com>