thunk: fix of malloc to g_new

Note that since thunking occurs throughout the lifetime of the QEMU
instance, there is no matching 'free' to correct.

Signed-off-by: Aarushi Mehta <mehta.aaru20@gmail.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Message-Id: <5310bd5d152fa36c1828a7cbd19fc893739d1609.camel@gmail.com>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>