This reduces unnecessary interrupts that host could send to guest while
guest is in the progress of irq handling.

If one vcpu is handling the irq, while another interrupt comes, in
handle_edge_irq(), the guest will mask the interrupt via mask_msi_irq()
which is a very heavy operation that goes all the way down to host.

 Here are some performance numbers on qemu:

 Before:
 -------------------------------------
   seq-read  : io=0 B, bw=269730KB/s, iops=67432 , runt= 62200msec
   seq-write : io=0 B, bw=339716KB/s, iops=84929 , runt= 49386msec
   rand-read : io=0 B, bw=270435KB/s, iops=67608 , runt= 62038msec
   rand-write: io=0 B, bw=354436KB/s, iops=88608 , runt= 47335msec
     clat (usec): min=101 , max=138052 , avg=14822.09, stdev=11771.01
     clat (usec): min=96 , max=81543 , avg=11798.94, stdev=7735.60
     clat (usec): min=128 , max=140043 , avg=14835.85, stdev=11765.33
     clat (usec): min=109 , max=147207 , avg=11337.09, stdev=5990.35
   cpu          : usr=15.93%, sys=60.37%, ctx=7764972, majf=0, minf=54
   cpu          : usr=32.73%, sys=120.49%, ctx=7372945, majf=0, minf=1
   cpu          : usr=18.84%, sys=58.18%, ctx=7775420, majf=0, minf=1
   cpu          : usr=24.20%, sys=59.85%, ctx=8307886, majf=0, minf=0
   vdb: ios=8389107/8368136, merge=0/0, ticks=19457874/14616506,
 in_queue=34206098, util=99.68%
  43: interrupt in total: 887320
 fio --exec_prerun="echo 3 > /proc/sys/vm/drop_caches" --group_reporting
 --ioscheduler=noop --thread --bs=4k --size=512MB --direct=1 --numjobs=16
 --ioengine=libaio --iodepth=64 --loops=3 --ramp_time=0
 --filename=/dev/vdb --name=seq-read --stonewall --rw=read
 --name=seq-write --stonewall --rw=write --name=rnd-read --stonewall
 --rw=randread --name=rnd-write --stonewall --rw=randwrite

 After:
 -------------------------------------
   seq-read  : io=0 B, bw=309503KB/s, iops=77375 , runt= 54207msec
   seq-write : io=0 B, bw=448205KB/s, iops=112051 , runt= 37432msec
   rand-read : io=0 B, bw=311254KB/s, iops=77813 , runt= 53902msec
   rand-write: io=0 B, bw=377152KB/s, iops=94287 , runt= 44484msec
     clat (usec): min=81 , max=90588 , avg=12946.06, stdev=9085.94
     clat (usec): min=57 , max=72264 , avg=8967.97, stdev=5951.04
     clat (usec): min=29 , max=101046 , avg=12889.95, stdev=9067.91
     clat (usec): min=52 , max=106152 , avg=10660.56, stdev=4778.19
   cpu          : usr=15.05%, sys=57.92%, ctx=77109411

, majf=0, minf=54
   cpu          : usr=26.78%, sys=101.40%, ctx=7387891, majf=0, minf=2
   cpu          : usr=19.03%, sys=58.17%, ctx=7681976, majf=0, minf=8
   cpu          : usr=24.65%, sys=58.34%, ctx=8442632, majf=0, minf=4
   vdb: ios=8389086/8361888, merge=0/0, ticks=17243780/12742010,
 in_queue=30078377, util=99.59%
  43: interrupt in total: 1259639
 fio --exec_prerun="echo 3 > /proc/sys/vm/drop_caches" --group_reporting
 --ioscheduler=noop --thread --bs=4k --size=512MB --direct=1 --numjobs=16
 --ioengine=libaio --iodepth=64 --loops=3 --ramp_time=0
 --filename=/dev/vdb --name=seq-read --stonewall --rw=read
 --name=seq-write --stonewall --rw=write --name=rnd-read --stonewall
 --rw=randread --name=rnd-write --stonewall --rw=randwrite

Signed-off-by: Asias He <asias@redhat.com>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>