build: add --enable/--disable-libudev

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>

configure

@@ -303,6 +303,7 @@ netmap="no"
 sdl="auto"
 sdl_image="auto"
 virtfs=""
+libudev="auto"
 mpath="auto"
 vnc="enabled"
 sparse="auto"
@@ -1002,6 +1003,10 @@ for opt do
   ;;
   --enable-virtfs) virtfs="yes"
   ;;
+  --disable-libudev) libudev="disabled"
+  ;;
+  --enable-libudev) libudev="enabled"
+  ;;
   --disable-mpath) mpath="disabled"
   ;;
   --enable-mpath) mpath="enabled"
@@ -1759,6 +1764,7 @@ disabled with --disable-FEATURE, default is enabled if available:
   vnc-png         PNG compression for VNC server
   cocoa           Cocoa UI (Mac OS X only)
   virtfs          VirtFS
+  libudev         Use libudev to enumerate host devices
   mpath           Multipath persistent reservation passthrough
   xen             xen backend driver support
   xen-pci-passthrough    PCI passthrough support for Xen
@@ -7060,7 +7066,7 @@ NINJA=$ninja $meson setup \
         -Dvnc=$vnc -Dvnc_sasl=$vnc_sasl -Dvnc_jpeg=$vnc_jpeg -Dvnc_png=$vnc_png \
         -Dgettext=$gettext -Dxkbcommon=$xkbcommon -Du2f=$u2f \
         -Dcapstone=$capstone -Dslirp=$slirp -Dfdt=$fdt \
-        -Diconv=$iconv -Dcurses=$curses \
+        -Diconv=$iconv -Dcurses=$curses -Dlibudev=$libudev\
         $cross_arg \
         "$PWD" "$source_path"