KVM: Coalesced MMIO support

KVM: Coalesced MMIO support

MMIO exits are more expensive in KVM or Xen than in QEMU because they
involve, at least, privilege transitions.  However, MMIO write
operations can be effectively batched if those writes do not have side
effects.

Good examples of this include VGA pixel operations when in a planar
mode.  As it turns out, we can get a nice boost in other areas too.
Laurent mentioned a 9.7% performance boost in iperf with the coalesced
MMIO changes for the e1000 when he originally posted this work for KVM.

Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
git-svn-id: svn://svn.savannah.nongnu.org/qemu/trunk@5961 c046a42c-6fe2-441c-8c8c-71466251a162