virtio-blk: emit udev event when device is resized

When virtio-blk device is resized from host (using block_resize from QEMU) emit
KOBJ_CHANGE uevent to notify guest about such change. This allows user to have
custom udev rules which would take whatever action if such event occurs. As a
proof of concept I've created simple udev rule that automatically resize
filesystem on virtio-blk device.

ACTION=="change", KERNEL=="vd*", \
        ENV{RESIZE}=="1", \
        ENV{ID_FS_TYPE}=="ext[3-4]", \
        RUN+="/sbin/resize2fs /dev/%k"
ACTION=="change", KERNEL=="vd*", \
        ENV{RESIZE}=="1", \
        ENV{ID_FS_TYPE}=="LVM2_member", \
        RUN+="/sbin/pvresize /dev/%k"

Signed-off-by: Milos Vyletel <milos.vyletel@sde.cz>
Tested-by: Asias He <asias@redhat.com>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au> (minor simplification)