ps2: Fix mouse stream corruption due to lost data

ps2: Fix mouse stream corruption due to lost data

This fixes an issue by adding bounds checking to multi-byte packets
where the PS/2 mouse data stream may become corrupted due to data being
discarded when the PS/2 ringbuffer is full.

Interrupts for Multi-byte responses are postponed until the final byte
has been queued.

These changes fix a bug where windows guests drop the mouse device
entirely requring the guest to be restarted.

Signed-off-by: Geoffrey McRae <geoff@hostfission.com>
Message-Id: <20180507150310.2FEA0381924@moya.office.hostfission.com>

[ kraxel: codestyle fixes ]

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>