libvirt.spec: Fix nbdkit selection logic on mingw and old rhel

libvirt.spec: Fix nbdkit selection logic on mingw and old rhel

rhel-8 lacks 'pidfd_open()' support and thus nbdkit can't be enabled
there.

mingw builds explicitly disable nbdkit support, but use
'--auto-features=enabled' thus omitting setting of
'nbdkit_config_default' results in meson thinking we want to enable it:

  ../meson.build:1018:2: ERROR: Problem encountered: nbdkit_config_default requires nbdkit to be enabled

Disable it explicitly. The meson logic might need to be fixed eventually
when switching it on by default.

Fixes: 9eabf14afbd429af1be195723fdeab5ccdd0b2cd
Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Andrea Bolognani <abologna@redhat.com>