tools: extend xc_assign_device() to support rdm reservation policy

tools: extend xc_assign_device() to support rdm reservation policy

This patch passes rdm reservation policy to xc_assign_device() so the policy
is checked when assigning devices to a VM.

Note this also bring some fallout to python usage of xc_assign_device().

CC: Ian Jackson <ian.jackson@eu.citrix.com>
CC: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
CC: Ian Campbell <ian.campbell@citrix.com>
CC: Wei Liu <wei.liu2@citrix.com>
CC: David Scott <dave.scott@eu.citrix.com>
Signed-off-by: Tiejun Chen <tiejun.chen@intel.com>
Acked-by: Wei Liu <wei.liu2@citrix.com>