All VGA devices share a common field subset; currently they do so by
a macro which defines the common fields inline their state structures,
relying on the the common state being placed at offset 0 in the structure.
This makes refactoring the code difficult and requires a lot of error prone
casts.

Replace the macro by a new VGACommonState structure, and the casts by
regular field access and container_of() for upcasts.

Signed-off-by: Avi Kivity <avi@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>