fw-cfg: support writeable blobs

fw-cfg: support writeable blobs

Useful to send guest data back to QEMU.

Changes from Laszlo Ersek <[email protected]>:
- rebase the patch from Michael Tsirkin's original postings at [1] and [2]
  to the following patches:
  - loader: Allow a custom AddressSpace when loading ROMs
  - loader: Add AddressSpace loading support to uImages
  - loader: fix handling of custom address spaces when adding ROM blobs
- reject such writes immediately that would exceed the end of the array,
  rather than performing a partial write before setting the error bit: see
  the (len != dma.length) condition
- document the write interface

[1] http://lists.nongnu.org/archive/html/qemu-devel/2016-02/msg04968.html
[2] http://lists.nongnu.org/archive/html/qemu-devel/2016-03/msg02735.html

Cc: "Gabriel L. Somlo" <[email protected]>
Cc: "Michael S. Tsirkin" <[email protected]>
Cc: Gerd Hoffmann <[email protected]>
Cc: Igor Mammedov <[email protected]>
Cc: Michael Walle <[email protected]>
Cc: Paolo Bonzini <[email protected]>
Cc: Peter Maydell <[email protected]>
Cc: Shannon Zhao <[email protected]>
Cc: [email protected]
Signed-off-by: Michael S. Tsirkin <[email protected]>
Signed-off-by: Laszlo Ersek <[email protected]>
Reviewed-by: Marcel Apfelbaum <[email protected]>
Acked-by: Gabriel Somlo <[email protected]>
Tested-by: Gabriel Somlo <[email protected]>
Reviewed-by: Michael S. Tsirkin <[email protected]>
Signed-off-by: Michael S. Tsirkin <[email protected]>
Reviewed-by: Eduardo Habkost <[email protected]>