xend: Fix 'xm pci_list_assignable_devices'
The current implementation of 'xm pci-list-assignable-devices' command
has a problem that it directly invokes hypercall using
xen.lowlevel.xc.
This is probably based on an assumption that the command is executed
on the host itself, but in fact there are cases xm commands can be
executed on a remote server through xmlrpc.
So this patch makes the xm command just inquire of xend about the
information of available devices.
Signed-off-by: Yosuke Iwamatsu <y-iwamatsu@ab.jp.nec.com>