x86/HVM: make explicit that hvm_print_line() does output only

x86/HVM: make explicit that hvm_print_line() does output only

On input "c" being 0xff should already have the effect of bailing early
(due to the isprint()), but let's rather make this explicit. Also
convert the BUG_ON() to an ASSERT() (nothing fatal happens in the
function if this is violated), at the same time extending what is being
checked.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>