e1000: flush queue whenever can_receive can go from false to true

e1000: flush queue whenever can_receive can go from false to true

When the guests replenish the receive ring buffer, the network device
should flush its queue of pending packets.  This is done with
qemu_flush_queued_packets.

e1000's can_receive can go from false to true when RCTL or RDT are
modified.

Reported-by: Luigi Rizzo <rizzo@iet.unipi.it>
Cc: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>
Cc: Jan Kiszka <jan.kiszka@siemens.de>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Amos Kong <akong@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>
(cherry picked from commit e8b4c680b41bd960ecccd9ff076b7b058e0afcd4)

Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>