libxc: introduce a per architecture scratch pfn for temporary grant mapping

libxc: introduce a per architecture scratch pfn for temporary grant mapping

The code to initialize the grant table in libxc uses
xc_domain_maximum_gpfn() + 1 to get a guest pfn for mapping the grant
frame and to initialize it.

This solution has two major issues:
    - The check of the return of xc_domain_maximum_gpfn is buggy because
    xen_pfn_t is unsigned and in case of an error -ERRNO is returned.
    Which is never catch with ( pfn <= 0 ).
    - The guest memory layout maybe filled up to the end, i.e
    xc_domain_maximum_gpfn() + 1 gives either 0 or an invalid PFN due to
    hardware limitation.

Futhermore, on ARM, xc_domain_maximum_gpfn() is not implemented and
return -ENOSYS. This will make libxc to use always the same PFN which
may colapse with an already mapped region (see xen/include/public/arch-arm.h
for the layout).

This patch only address the problem for ARM, the x86 version use the same
behavior (ie xc_domain_maximum_gpfn() + 1), as I'm not familiar with Xen x86.

A new function xc_core_arch_get_scratch_gpfn is introduced to be able to
choose the gpfn per architecture.

For the ARM version, we use the GUEST_GNTTAB_GUEST which is the base of
the region by the guest to map the grant table. At the build time,
nothing is mapped there.

At the same time correctly check the return of xc_domain_maximum_gpfn
for x86.

Signed-off-by: Julien Grall <julien.grall@linaro.org>
Cc: Jan Beulich <jbeulich@suse.com>
Cc: Andrew Cooper <andrew.cooper3@citrix.com>
Cc: Roger Pau Monné <roger.pau@citrix.com>
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>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>