tap: Correctly handle frames of odd length

The Qemu socket protocol consists of a 32-bit frame length in network (BE)
order, followed by the Ethernet frame itself.  As far as I can tell,
frames can be any length, with no particular alignment requirement.  This
means that although pkt_buf itself is aligned, if we have a frame of odd
length, frames after it will have their frame length at an unaligned
address.

Currently we load the frame length by just casting a char pointer to
(uint32_t *) and loading.  Some platforms will generate a fatal trap on
such an unaligned load.  Even if they don't casting an incorrectly aligned
pointer to (uint32_t *) is undefined behaviour, strictly speaking.

Introduce a new helper to safely load a possibly unaligned value here.  We
assume that the compiler is smart enough to optimize this into nothing on
platforms that provide performant unaligned loads.  If that turns out not
to be the case, we can look at improvements then.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Stefano Brivio <sbrivio@redhat.com>

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