xen: Look for Xen notes in section headers too

xen: Look for Xen notes in section headers too

Mirror behaviour of ELF loader in libxc: first look for Xen notes in
PT_NOTE segment, then in SHT_NOTE section and only then fallback to
a section with __xen_guest name. This fixes loading PV kernels that
Xen note have outside of PT_NOTE. While this may be result of a buggy
linker script, loading such kernel directly works fine, so make it work
with GRUB too. Specifically, this applies to binaries built from Unikraft.

Signed-off-by: Marek Marczykowski-Górecki <marmarek@invisiblethingslab.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>