x86: don't allow Dom0 to map MSI-X table writably

x86: don't allow Dom0 to map MSI-X table writably

With the traditional qemu tree fixed to not use PROT_WRITE anymore in
the mmap() call for this region, and with the upstream qemu tree not
being capable of handling passthrough, yet, there's no need to treat
Dom specially here anymore.

This continues to leave unaddressed the case where PV guests map the
MSI-X table page(s) before setting up the first MSI-X interrupt (see
the original c/s 22182:68cc3c514a0a description for options).

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Keir Fraser <keir@xen.org>