ui/cocoa.m: prevent stuck key situation

When the user puts QEMU in the background while holding
down a key, QEMU will not receive the keyup event when
the user lets go of the key. When the user goes back to
QEMU, QEMU will think the key is still down causing
stuck key symptoms. This patch fixes this problem by
releasing all down keys when QEMU goes into the
background.

Signed-off-by: John Arbuckle <programmingkidx@gmail.com>
Message-id: 7A3FA6EE-84C8-4422-A786-C899B7229D32@gmail.com
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
