libxl: Fix segfault in get_all_assigned_devices

libxl: Fix segfault in get_all_assigned_devices

pcidevs is an array of ndev elements (ndev is the number of pci devices
assigend to a specific domain), but we access pcidevs + *num
where *num is the global number of pci devices assigned so far to all
domains in the system.

Fix the issue removing pcidevs and just realloc'ing *list every time we
want to add a new pci device to the array.

Signed-off-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Committed-by: Ian Jackson <ian.jackson@eu.citrix.com>