xen: fix parse_bool() with empty string

xen: fix parse_bool() with empty string

parse_bool() should return -1 in case it is called with an empty
string. In order to allow boolean parameters in the cmdline without
specifying a value this case must be handled in _cmdline_parse() by
always passing a value string.

This fixes commit 532dec8e31174ed450adfd36a4b0b41dec27010d ("xen:
add an optional string end parameter to parse_bool()")

Reported-by: Andrew Cooper <andrew.cooper3@citrix.com>
Signed-off-by: Juergen Gross <jgross@suse.com>
Acked-by: Wei Liu <wei.liu2@citrix.com>
Tested-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>