libxl: remove existence check for PCI device hotplug

libxl: remove existence check for PCI device hotplug

The existence check is to make sure a device is not added to a guest
multiple times.

PCI device backend path has different rules from vif, disk etc. For
example:
/local/domain/0/backend/pci/9/0/dev-1/0000:03:10.1
/local/domain/0/backend/pci/9/0/key-1/0000:03:10.1
/local/domain/0/backend/pci/9/0/dev-2/0000:03:10.2
/local/domain/0/backend/pci/9/0/key-2/0000:03:10.2

The devid for PCI devices is hardcoded 0. libxl__device_exists only
checks up to /local/.../9/0 so it always returns true even the device is
assignable.

Remove invocation of libxl__device_exists. We're sure at this point that
the PCI device is assignable (hence no xenstore entry or JSON entry).
The check is done before hand. For HVM guest it's done by calling
xc_test_assign_device and for PV guest it's done by calling
pciback_dev_is_assigned.

Reported-by: Li, Liang Z <liang.z.li@intel.com>
Signed-off-by: Wei Liu <wei.liu2@citrix.com>
Cc: Ian Campbell <ian.campbell@citrix.com>
Cc: Ian Jackson <ian.jackson@eu.citrix.com>
Cc: Konrad Wilk <konrad.wilk@oracle.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>