pvh: Add x86/HVM direct boot ABI header file

pvh: Add x86/HVM direct boot ABI header file

The x86/HVM direct boot ABI permits Qemu to be able to boot directly
into the uncompressed Linux kernel binary with minimal firmware involvement.

	https://xenbits.xen.org/docs/unstable/misc/pvh.html

This commit adds the header file that defines the start_info struct
that needs to be populated in order to use this ABI.

The canonical version of start_info.h is in the Xen codebase.
(like QEMU, the Linux kernel uses a copy as well).

Signed-off-by: Liam Merwick <Liam.Merwick@oracle.com>
Reviewed-by: Konrad Rzeszutek Wilk <Konrad.Wilk@oracle.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>