Initially, libudev detection was bundled with --enable-mpath because
qemu-pr-helper was the only user of libudev.  Recently however the USB
U2F emulation has also started using libudev, so add a separate
option.  This also allows 1) disabling libudev if desired for static
builds and 2) for non-static builds, requiring libudev even if
multipath support is undesirable.

The multipath test is adjusted, because it is now possible to enter it
with configurations that should fail, such as --static --enable-mpath
--disable-libudev.

Reported-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>