fix endianness problem sharing the videoram buffer

fix endianness problem sharing the videoram buffer

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This patch fixes vga rendering when the guest endianness differs from
the host endianness: in this case we can only share the buffer if the
bpp is 32 and we must change the pixelformat accordingly.

Signed-off-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
git-svn-id: svn://svn.savannah.nongnu.org/qemu/trunk@6413 c046a42c-6fe2-441c-8c8c-71466251a162