qemu-char: add support for U-prefixed symbols

This patch adds support for Unicode symbols in keymap files. This
feature was already used in some keyboard layouts in QEMU generated
from XKB (e.g. Arabic) but it wasn't implemented in QEMU source code.

There is no need for check of validity of the hex string after U character
because strtol returns 0 in case the conversion was unsuccessful.

Signed-off-by: Jan Krupa <jkrupa@suse.com>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>
