VMware: add support for driver method instance_exists

The default driver implementation is terribly expensive. This
overrides the implementation with a far more optimum solution.

A similar implementation exists in xen and libvirt drivers.

Change-Id: I656298564077701683a175ca1d1fe788328d0c74
VMware: Fix race in spawn() when resizing cached image

spawn() guards against multiple threads simultaneously attempting to
cache the same image, but it wasn't guarding against them
simultanously trying to create a resized copy in the cache. Attempting
to create a large number of images simultaneously of an uncached image
would result in a race to create the resized image. This resulted in 2
classes of failed instance:

1. Instances whose disk was a linked clone of a copy which had been
   subsequently overwritten. These were corrupt.
2. Instances whose spawn() failed in ExtendVirtualDisk_Task due to a
   locked image.

This patch creates a Nova-local lock for the resized image. The image
is in a per-Nova directory on the datastore, so inter-Nova locking is
not a concern. The lock guards both testing for the existence of the
image, and its creation. Therefore when multiple processes race, only
1 will create the resized copy, and all others will find and use it.
In normal usage this will add the overhead of an additional
uncontended local lock creation and deletion in spawn().

In wrapping this code in a lock, we also make certain that any failure
to create the resized image is appropriately cleaned up. Otherwise
subsequent users will attempt to use a corrupt copy.

Change-Id: I3df3d614656e511c909b6c1837582c0d34bf84c6
Closes-bug: 1333587
Merge "VMware: add support for driver method instance_exists"
Merge "VMware: Fix race in spawn() when resizing cached image"