QEMU_PACKED results in a MinGW compiler warning when it is
used for single structure elements:

warning: 'gcc_struct' attribute ignored

Using QEMU_PACKED for the whole structure avoids the compiler warning
without changing the memory layout.
Signed-off-by: NStefan Weil <sw@weilnetz.de>
Signed-off-by: NStefan Hajnoczi <stefanha@linux.vnet.ibm.com>