mini-os: do not wait for pci backend in pcifront_scan

mini-os: do not wait for pci backend in pcifront_scan

This blocks the main thread indefinitely if there is no PCI backend present in
xenstore.

Even in the case where there are passthrough devices configured libxl creates
the stubdom and waits for it to startup _before_ adding the backend. Since the
stub domains main thread is blocked before it can write the "running" state to
xenstore the toolstack eventually times out and kills everything.

There is already a separate pcifront thread which waits for the backend to
appear and calls init_pcifront at the appropriate time should a backend ever
appear.

Unfortunately I don't have any free test boxes with VT-d so I haven't been able
to test the cases where PCI deivces are passed through but I obviously have
tested that I can now start an HVM domain with stub qemu without PCI devices
passed through which I couldn't do before so this is an improvement. This stuff
is a bit like pushing the lump around the carpet :-/

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
Acked-by: Ian Jackson <ian.jackson.citrix.com>
Committed-by: Ian Jackson <ian.jackson.citrix.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>