xen: fix two errors when debug is enabled

xen: fix two errors when debug is enabled

This patch fixes:
1. build error in xen_pt.c when XEN_PT_LOGGING_ENABLED is defined
2. debug output format string error when DEBUG_XEN is defined

In the second case I also have the output info in consistent with the
output in mapping function - that is, print start_addr instead of
phys_offset.

Signed-off-by: Sander Eikelenboom <linux@eikelenboom.it>
Signed-off-by: Wei Liu <wei.liu2@citrix.com>
Signed-off-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>