qemu: build command line for the TPM Proxy device

qemu: build command line for the TPM Proxy device

This patch wraps it up all the wiring done in previous patches,
enabling a PPC64 guest to launch a guest using a TPM Proxy
device.

Note that device validation is already being done in qemu_validate.c,
qemuValidateDomainDeviceDefTPM(), on domain define time. We don't
need to verify QEMU capabilities for this device again inside
qemu_command.c.

Tested-by: Satheesh Rajendran <sathnaga@linux.vnet.ibm.com>
Reviewed-by: Stefan Berger <stefanb@linux.ibm.com>
Signed-off-by: Daniel Henrique Barboza <danielhb413@gmail.com>
Signed-off-by: Ján Tomko <jtomko@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>