passthrough/stubdom: clean up hypercall privilege checking

passthrough/stubdom: clean up hypercall privilege checking

This patch adds securty checks for pci passthrough related hypercalls
to enforce that the current domain owns the resources that it is about
to remap. It also adds a call to xc_assign_device to xend and removes
the PRIVILEGED_STUBDOMS flags.

Signed-off-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>