libxl: Introduce pci_assignable_add and pci_assignable_remove

Introduce libxl helper functions to prepare devices to be passed
through to guests.  This is meant to replace of all the manual sysfs
commands which are currently required.

pci_assignable_add accepts a BDF for a device and will:
* Unbind a device from its current driver, if any
* If "rebind" is set, it will store the path of the driver from which we
  unplugged it in /libxl/pciback/$BDF/driver_path
* If create a slot for it in pciback if one doesn't yet exist
* Bind the device to pciback

At this point it will show up in pci_assignable_list, and is ready to
be passed through to a guest.

pci_assignable_remove accepts a BDF for a device and will:
* Unbind the device from pciback
* Remove the slot from pciback
* If "rebind" is set, and /libx/pciback/$BDF/driver_path exists, it
  will attempt to rebind the device to its original driver.

Both functions are idempotent: if the desired end state has already
been reached, they return SUCCESS.

NB that "$BDF" in this case uses '-' instead of ':' and '.', because
':' and '.' are illegal characters in xenstore paths.

Signed-off-by: George Dunlap <george.dunlap@eu.citrix.com>
Committed-by: Ian Campbell <ian.campbell@citrix.com>

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