libxenlight: move logging macros to the public header

libxenlight: move logging macros to the public header

This patch moves the logging macros to the public header so that they
can be reused by the client of the library.  It also refactors the
code to create the qemu logfile into a generic function that can be
reused to create generic xen logfiles under /var/log/xen.  Finally xl
is changed to log to file when running in background.

Signed-off-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>