uuid: Make qemu_uuid_bswap() take and return a QemuUUID

uuid: Make qemu_uuid_bswap() take and return a QemuUUID

Currently qemu_uuid_bswap() takes a pointer to the QemuUUID to
be byte-swapped. This means it can't be used when the UUID
to be swapped is in a packed member of a struct. It's also
out of line with the general bswap*() functions we provide
in bswap.h, which take the value to be swapped and return it.

Make qemu_uuid_bswap() take a QemuUUID and return the swapped version.

This fixes some clang warnings about taking the address of
a packed struct member in block/vdi.c.

Signed-off-by: Peter Maydell <[email protected]>
Reviewed-by: Michael S. Tsirkin <[email protected]>
Reviewed-by: Marc-André Lureau <[email protected]>
Signed-off-by: Kevin Wolf <[email protected]>