target-arm: Avoid g_hash_table_get_keys()

g_hash_table_get_keys() was only introduced in glib 2.14, and we're
still targeting a minimum version of 2.12.  Rewrite the offending
code (introduced in commit 721fae12) to use g_hash_table_foreach()
to build the list of keys.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Tested-by: Laurent Desnogues <laurent.desnogues@gmail.com>
Tested-by: Peter Crosthwaite <peter.crosthwaite@xilinx.com>
Message-id: 1372678819-8633-1-git-send-email-peter.maydell@linaro.org