OvmfPkg/XenResetVector: Saving start of day pointer for PVH guests

OvmfPkg/XenResetVector: Saving start of day pointer for PVH guests

As described in the Xen PVH documentation [1], "ebx: contains the
physical memory address where the loader has placed the boot start info
structure". To have this pointer saved to be able to use it later in the
PEI phase, we allocate some space in the MEMFD for it. We use 'XPVH' as
a signature (for "Xen PVH").

[1] https://xenbits.xenproject.org/docs/unstable/misc/pvh.html

Ref: https://bugzilla.tianocore.org/show_bug.cgi?id=1689
Signed-off-by: Anthony PERARD <anthony.perard@citrix.com>
Acked-by: Laszlo Ersek <lersek@redhat.com>
Message-Id: <20190813113119.14804-8-anthony.perard@citrix.com>