xen/balloon: xen_shim_domain() support

xen/balloon: xen_shim_domain() support

Xen ballooning uses hollow struct pages (with the underlying PFNs being
populated/unpopulated via hypercalls) which are used by the grant logic
to map grants from other domains.

For purposes of a KVM based xen-shim, this model is not useful --
mapping is unnecessary since all guest memory is already mapped in the
KVM host. The simplest option is to just translate grant references to
GPAs (essentially a get_page() on the appropriate GPA.)

This patch provides an alternate balloon allocation mechanism where in
the allocation path we just provide a constant struct page
(corresponding to page 0.) This allows the calling code -- which does a
page_to_pfn() on the returned struct page -- to remain unchanged before
doing the grant operation (which in this case would fill in the real
struct page.)

Co-developed-by: Ankur Arora <ankur.a.arora@oracle.com>
Signed-off-by: Joao Martins <joao.m.martins@oracle.com>
Signed-off-by: Ankur Arora <ankur.a.arora@oracle.com>