kvm tools: Add ioeventfd support

kvm tools: Add ioeventfd support

ioeventfd is way provided by KVM to receive notifications about
reads and writes to PIO and MMIO areas within the guest.

Such notifications are usefull if all we need to know is that
a specific area of the memory has been changed, and we don't need
a heavyweight exit to happen.

The implementation uses epoll to scale to large number of ioeventfds.

Benchmarks ran on a seperate (non boot) 1GB virtio-blk device, formatted
as ext4, using bonnie++.

cmd line:
# bonnie++ -d temp/ -c 2 -s 768 -u 0

Before:
Version  1.96       ------Sequential Output------ --Sequential Input- --Random-
Concurrency   2     -Per Chr- --Block-- -Rewrite- -Per Chr- --Block-- --Seeks--
Machine        Size K/sec %CP K/sec %CP K/sec %CP K/sec %CP K/sec %CP  /sec %CP
tux            768M   498  99 381127  74 269712  48  2871  99 717109  50 +++++ +++
Latency             18368us   31063us   21281us    3017us    6875us     251ms
Version  1.96       ------Sequential Create------ --------Random Create--------
tux                 -Create-- --Read--- -Delete-- -Create-- --Read--- -Delete--
              files  /sec %CP  /sec %CP  /sec %CP  /sec %CP  /sec %CP  /sec %CP
                 16 +++++ +++ +++++ +++ +++++ +++ +++++ +++ +++++ +++ +++++ +++
Latency               148us     588us    2792us    1547us    1543us     218us

After:
Version  1.96       ------Sequential Output------ --Sequential Input- --Random-
Concurrency   2     -Per Chr- --Block-- -Rewrite- -Per Chr- --Block-- --Seeks--
Machine        Size K/sec %CP K/sec %CP K/sec %CP K/sec %CP K/sec %CP  /sec %CP
tux            768M   499  99 459779  73 350689  54  2997  99 860395  58 +++++ +++
Latency             17194us   14619us   26358us    4055us    7890us   44122us
Version  1.96       ------Sequential Create------ --------Random Create--------
tux                 -Create-- --Read--- -Delete-- -Create-- --Read--- -Delete--
              files  /sec %CP  /sec %CP  /sec %CP  /sec %CP  /sec %CP  /sec %CP
                 16 +++++ +++ +++++ +++ +++++ +++ +++++ +++ +++++ +++ +++++ +++
Latency               135us     567us    2564us     134us    1500us     231us

Signed-off-by: Sasha Levin <levinsasha928@gmail.com>
Signed-off-by: Pekka Enberg <penberg@kernel.org>