libxl/PCI: Fix PV hotplug & stubdom coldplug

libxl/PCI: Fix PV hotplug & stubdom coldplug

commit 0fdb48ffe7a1 "libxl: Make sure devices added by pci-attach are
reflected in the config" broken PCI hotplug (xl pci-attach) for PV
domains when it moved libxl__create_pci_backend() later in the function.

This also broke HVM + stubdom PCI passthrough coldplug.  For that, the
PCI devices are hotplugged to a running PV stubdom, and then the QEMU
QMP device_add commands are made to QEMU inside the stubdom.

A running PV domain calls libxl__wait_for_backend().  With the current
placement of libxl__create_pci_backend(), the path does not exist and
the call immediately fails:
libxl: error: libxl_device.c:1388:libxl__wait_for_backend: Backend /local/domain/0/backend/pci/43/0 does not exist
libxl: error: libxl_pci.c:1764:device_pci_add_done: Domain 42:libxl__device_pci_add failed for PCI device 0:2:0.0 (rc -3)
libxl: error: libxl_create.c:1857:domcreate_attach_devices: Domain 42:unable to add pci devices

The wait is only relevant when:
1) The domain is PV
2) The domain is running
3) The backend is already present

This is because:

1) xen-pcifront is only used for PV.  It does not load for HVM domains
   where QEMU is used.

2) If the domain is not running (starting), then the frontend state will
   be Initialising.  xen-pciback waits for the frontend to transition to
   at Initialised before attempting to connect.  So a wait for a
   non-running domain is not applicable as the backend will not
   transition to Connected.

3) For presence, num_devs is already used to determine if the backend
   needs to be created.  Re-use num_devs to determine if the backend
   wait is necessary.  The wait is necessary to avoid racing with
   another PCI attachment reconfiguring the front/back or changing to
   some other state like closing.  If we are creating the backend, then
   we don't have to worry about the state since it is being created.

Fixes: 0fdb48ffe7a1 ("libxl: Make sure devices added by pci-attach are
reflected in the config")

Signed-off-by: Jason Andryuk <jandryuk@gmail.com>
Reviewed-by: Paul Durrant <paul@xen.org>
Reviewed-by: Anthony PERARD <anthony.perard@citrix.com>