tools: Disable virt-login-shell on mingw

tools: Disable virt-login-shell on mingw

So, after bec787ee9da we are building virt-login-shell
independent of LXC driver. This is nice, but the binary is
enabled by default which makes no sense on mingw. In fact, it
triggers some compilation errors there:

  CC       virt_login_shell-virt-login-shell.o
../../tools/virt-login-shell.c: In function 'main':
../../tools/virt-login-shell.c:289:15: error: implicit declaration of function 'sysconf' [-Werror=implicit-function-declaration]
     openmax = sysconf(_SC_OPEN_MAX);
               ^
../../tools/virt-login-shell.c:289:5: error: nested extern declaration of 'sysconf' [-Werror=nested-externs]
     openmax = sysconf(_SC_OPEN_MAX);
     ^
../../tools/virt-login-shell.c:289:23: error: '_SC_OPEN_MAX' undeclared (first use in this function)
     openmax = sysconf(_SC_OPEN_MAX);
                       ^
../../tools/virt-login-shell.c:289:23: note: each undeclared identifier is reported only once for each function it appears in
cc1: all warnings being treated as errors

While we could workaround sysconf(_SC_OPEN_MAX) issue, the binary
itself makes no sense on systems where no LXC can be spawned.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>