libvhost-user: Use unsigned int i for some for-loop iterations

libvhost-user: Use unsigned int i for some for-loop iterations

The sign-compare warning also hits some of the for-loops, but it easy
fixed by just making the iterator variable unsigned int.

  CC       libvhost-user.o
libvhost-user.c: In function ‘vu_gpa_to_va’:
libvhost-user.c:223:19: error: comparison of integer expressions of different signedness: ‘int’ and ‘uint32_t’ {aka ‘unsigned int’} [-Werror=sign-compare]
  223 |     for (i = 0; i < dev->nregions; i++) {
      |                   ^

Signed-off-by: Marcel Holtmann <marcel@holtmann.org>
Message-Id: <decb925e1a6fb9538738d2570bda2804f888fa15.1671741278.git.marcel@holtmann.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>