libxl_pci: Only check if qemu-dm is running in qemu-trad case

libxl_pci: Only check if qemu-dm is running in qemu-trad case

QEMU upstream (or qemu-xen) may not have set "running" state in
xenstore. "running" with QEMU doesn't mean that the binary is
running, it means that the emulation have started. When adding a
pci-passthrough device to QEMU, we do so via QMP, we have a direct
answer to whether QEMU is running or not, no need to check ahead.

Moving the check to do it only with qemu-trad makes upcoming changes
simpler.

Signed-off-by: Anthony PERARD <anthony.perard@citrix.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>