KVM: Fix page-crossing MMIO

MMIO that are split across a page boundary are currently broken - the
code does not expect to be aborted by the exit to userspace for the
first MMIO fragment.

This patch fixes the problem by generalizing the current code for handling
16-byte MMIOs to handle a number of "fragments", and changes the MMIO
code to create those fragments.

Signed-off-by: Avi Kivity <avi@redhat.com>
Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>