qemu-io: Implement bdrv_load_vmstate/bdrv_save_vmstate

The load_vmstate and save_vmstate functions are implemented as a
variation of the normal read/write operation, enabled by the -b option.
This is the same mechanism as is used to switch from read/write to
pread/pwrite.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>