libvhost-user: fix -Werror=format= warnings with __u64 fields

libvhost-user: fix -Werror=format= warnings with __u64 fields

../subprojects/libvhost-user/libvhost-user.c:1070:12: error: format ‘%lx’ expects argument of type ‘long unsigned int’, but argument 3 has type ‘__u64’ {aka ‘long long unsigned int’} [-Werror=format=]
 1070 |     DPRINT("    desc_user_addr:   0x%016" PRIx64 "\n", vra->desc_user_addr);
      |            ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~              ~~~~~~~~~~~~~~~~~~~
      |                                                           |
      |                                                           __u64 {aka long long unsigned int}

Rather than using %llx, which may fail if __u64 is declared differently
elsewhere, let's just cast the values. Feel free to propose a better solution!

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-Id: <20210505151313.203258-2-marcandre.lureau@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>