OvmfPkg: Introduce XenIoPvhDxe to initialize Grant Tables

OvmfPkg: Introduce XenIoPvhDxe to initialize Grant Tables

XenIoPvhDxe use XenIoMmioLib to reserve some space to be use by the
Grant Tables.

The call is only done if it is necessary, we simply detect if the
guest is PVH, as in this case there is currently no PCI bus, and no
PCI Xen platform device which would start the XenIoPciDxe and allocate
the space for the Grant Tables.

Ref: https://bugzilla.tianocore.org/show_bug.cgi?id=1689
Signed-off-by: Anthony PERARD <anthony.perard@citrix.com>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>
Message-Id: <20190813113119.14804-34-anthony.perard@citrix.com>