virtio: Implement MMIO based virtio transport

virtio: Implement MMIO based virtio transport

Add support for the generic MMIO based virtio transport.

This patch includes some fixes for bugs spotted by
Ying-Shiuan Pan <yspan@itri.org.tw>.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: KONRAD Frederic <fred.konrad@greensocs.com>
Message-id: 1373977512-28932-6-git-send-email-peter.maydell@linaro.org
[Fred changes: updated to new virtio-bus mechanisms]
Signed-off-by: KONRAD Frederic <fred.konrad@greensocs.com>
[PMM changes:
 * fixed trivial makefile conflict
 * removed unused int_enable
 * host_features doesn't need migrating
 * reset guest accessible state in the reset function
 * minor style fixes like extra blank lines
 * RAZ/WI if there's no backend
 * made transport size 0x200, in line with kvmtool
 * set has_variable_vring_alignment
]