libvhost-user: Cast rc variable to avoid compiler warning

libvhost-user: Cast rc variable to avoid compiler warning

The assert from recvmsg() return value against an uint32_t size field
from a protocol struct throws a compiler warning.

  CC       libvhost-user.o
In file included from libvhost-user.c:27:
libvhost-user.c: In function ‘vu_message_read_default’:
libvhost-user.c:363:19: error: comparison of integer expressions of different signedness: ‘int’ and ‘uint32_t’ {aka ‘unsigned int’} [-Werror=sign-compare]
  363 |         assert(rc == vmsg->size);
      |                   ^~

This is not critical, but annoying when the libvhost-user source are
used in an external project that has this compiler warning switched on.

Signed-off-by: Marcel Holtmann <marcel@holtmann.org>
Message-Id: <7a791e27b7bd3e0a8b8cc8fbb15090a870d226d5.1671741278.git.marcel@holtmann.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>