build: avoid ld_preload tests on mingw
    
    Running ./autobuild.sh complained during the mingw cross-compile:
    
      CC       libvirportallocatormock_la-virportallocatortest.lo
    ../../tests/virportallocatortest.c:32:20: fatal error: dlfcn.h: No such file or directory
     # include <dlfcn.h>
                        ^
    compilation terminated.  With that fixed, the next failure was:
    
      CCLD     qemuxml2argvmock.la
    libtool: link: libtool library `qemuxml2argvmock.la' must begin with `lib'
    libtool: link: Try `libtool --help --mode=link' for more information.
    
    While we don't need to limit all LD_PRELOAD tests to just Linux, we
    do need to limit them to platforms that actually support loading;
    we also need to avoid building qemu tests when qemu is not enabled.
    
    * tests/virportallocatortest.c: Make conditional on <dlfcn.h>.
    * tests/Makefile.am (test_libraries): Only build qemu mock library
    when building qemu tests.
    
    Signed-off-by: Eric Blake <eblake@redhat.com>