Revert "target-arm: Avoid g_hash_table_get_keys()"

Since we now require GLib 2.22+ (commit f40685c6), we don't have to
work around lack of g_hash_table_get_keys() anymore.

This reverts commit 82a3a118.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-id: 1432749090-4698-1-git-send-email-armbru@redhat.com
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>