scsi: make io_timeout configurable

The current code sets an infinite timeout on SG_IO requests,
causing the guest to stall if the host experiences a frame
loss.
This patch adds an 'io_timeout' parameter for SCSIDevice to
make the SG_IO timeout configurable, and also shortens the
default timeout to 30 seconds to avoid infinite stalls.

Signed-off-by: Hannes Reinecke <hare@suse.de>
Message-Id: <20201116183114.55703-3-hare@suse.de>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>