libxl: libxl__device_pci_destroy_all should succeed if there is no PCI bus

libxl: libxl__device_pci_destroy_all should succeed if there is no PCI bus

Since 23915:4c4b72c94bac "" it is possible that no bus exists and this is not
an error. Removes the following spurious warning when destroying a PV domain
with no PCI devices:
    libxl: error: libxl.c:759:libxl_domain_destroy: pci shutdown failed
for domid 1005

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
Acked-by: Roger Pau Monne <roger.pau@entel.upc.edu>
Committed-by: Ian Jackson <ian.jackson@eu.citrix.com>