virtio: Clear interrupt status register in virtio-blk

virtio: Clear interrupt status register in virtio-blk

The VRING_AVAIL_F_NO_INTERRUPT flag is a hint that interrupts should be
suppressed.  It does not guarantee that interrupts will not be raised.
Therefore, make sure to clear the interrupt after each virtio-blk read.
This avoids a stuck interrupt interfering with the OS loaded later in
the boot process.

Signed-off-by: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>