p2v: wait for qemu-nbd before starting conversion (RHBZ#1167774)

Wait up to 10 seconds for qemu-nbd to start up and respond to clients.
Otherwise the conversion server may attempt to connect before qemu-nbd
is ready to serve it.
resize: fix 'No space left on device' problem when copying to an extended partition (RHBZ#1169015)

Because of the size of an extended partition reported by Linux is always 1024
bytes, so it will always fail to copy to an extended partition.

This patch fixes this problem by copying to the offset of an extended
partition in the destination disk.

Signed-off-by: Hu Tao <hutao@cn.fujitsu.com>