Add new macro QEMU_PACKED for packed C structures

A packed struct needs different gcc attributes for compilations
with MinGW compilers because glib-2.0 adds compiler flag
-mms-bitfields which modifies the packing algorithm.

Attribute gcc_struct reverses the negative effects of -mms-bitfields.
QEMU_PACKED sets this attribute and must be used for any packed
struct which is affected by -mms-bitfields.

Signed-off-by: Stefan Weil <weil@mail.berlios.de>
Signed-off-by: Blue Swirl <blauwirbel@gmail.com>
