Add optional log level settings

Add optional log level settings

Adds XenLogLevel and QemuLogLevel to xen.sys and ConsoleLogLevel
to xenbus.sys, which can be used to override the default log levels.
Each value is a REG_MULTI_SZ that contains an enumerated list of
log levels; TRACE, INFO, WARNING, ERROR, CRITICAL are defined.

Signed-off-by: Owen Smith <owen.smith@citrix.com>
Minor style tweaks.

Signed-off-by: Paul Durrant <paul.durrant@citrix.com>