libxc: Defer initialization of start_page for HVM guests

libxc: Defer initialization of start_page for HVM guests

With commit 8c45adec18e0 ("libxc: create unmapped initrd in domain
builder if supported") location of ramdisk may not be available to
HVMlite guests by the time alloc_magic_pages_hvm() is invoked if the
guest supports unmapped initrd.

So let's move ramdisk info initialization (along with a few other
operations that are not directly related to allocating magic/special
pages) from alloc_magic_pages_hvm() to bootlate_hvm().

Since we now split allocation and mapping of the start_info segment
let's stash it, along with cmdline length, in xc_dom_image so that we
can check whether we are mapping correctly-sized range.

We can also stop using xc_dom_image.start_info_pfn and leave it for
PV(H) guests only.

Signed-off-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Acked-by: Roger Pau Monné <roger.pau@citrix.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>