vmw_pvscsi: use PCI DMA APIs

It is wrong to use address_space_memory directly, because there could be an
IOMMU in the middle.  Passing the entire PVSCSIRingInfo to RS_GET_FIELD
and RS_SET_FIELD makes it easy to go back to the PVSCSIState.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>