target-i386: Use switch in check_hw_breakpoints()

Replace an if statement using magic numbers for breakpoint type with a
more explicit switch statement. This is to aid readability.

Change the return type and force_dr6_update argument type to bool.

While at it, fix Coding Style issues (missing braces).

Signed-off-by: liguang <lig.fnst@cn.fujitsu.com>
Signed-off-by: Andreas Färber <afaerber@suse.de>
