hw/arm/virt: Rename 'vbi' variables to 'vms'

Rename all the variables which used to be VirtBoardInfo*
and are now VirtMachineState* so their names are in line
with the type being used.

Apart from the removal of the line 'VirtMachineState *vbi = vms;'
this commit is purely a search-and-replace of 'vbi' with 'vms'.
Signed-off-by: NPeter Maydell <peter.maydell@linaro.org>
Reviewed-by: NAndrew Jones <drjones@redhat.com>