qemu_process: introduce qemuProcessCreatePretendCmd

qemu_process: introduce qemuProcessCreatePretendCmd

This will skip few steps from qemuProcessStart in order to create only
qemu CMD.  Use a VIR_QEMU_PROCESS_START_PRETEND for all the qemuProcess*
functions called by this one to not modify or check host.

This new function will be used later on for XMLToNative API and also for
qemuxml2argvtest to make sure that both API and test uses the same code
as qemuProcessStart.

We need also update qemuProcessInit to wrap few lines of code with check
that VIR_QEMU_PROCESS_START_PRETEND that makes sense only for
qemuProcessStart.

Signed-off-by: Pavel Hrdina <phrdina@redhat.com>