Resize block device after swap to larger volume

After swap to larger volume, instance's block device should be
resized. Otherwise, the guest can't utilize the extra space.

This patch make the libvirt driver resize block device after mirror
the volume. And add new parameter 'resize_to' to virt driver, that
is used to indicate the new size.

Change-Id: Ib4d65e8812c7d3c28100155124218c75a94e16e7
Closes-Bug: #1341459
Merge "Resize block device after swap to larger volume"