Removes unnecessary qemu-img dependency on powervm driver

Fixes bug 1131033

Power VM driver is using qemu-img through the fetch_to_raw call
No need to attempt a conversion since power images can only
be represented with the raw format, removing thus the need for
any conversions by calling fetch instead of fetch_to_raw.

Change-Id: I94b46e184cb87924a4754a0a48d9c2013a3eb756
Make guestfs use same libvirt URI as Nova.

Updates our libguestfs vfs implementation so that it makes use of
the same libvirt connection URI as Nova does when using
attach mode 'libvirt'. Previously when libguestfs used attach
mode 'libvirt' (the default on some newer distros like Fedora 18)
you could get permissions issues and ultimately get a libvirt
connection error from libguestfs:

  could not connect to libvirt (URI = NULL)

Docs on guestfs attach method can be found here:

  http://libguestfs.org/guestfs.3.html#attach-method

Having libguestfs use the same connection URL as Nova makes sense
and gets past these permission issues. (avoiding things like forcing
you to create a $HOME dir for the nova user among other things)

Full thread on this is here:

  https://bugzilla.redhat.com/show_bug.cgi?id=913345

Note that on most distros the default libguestfs attach method
is still 'appliance' so this would have no affect unless explicitly
enabled via LIBGUESTFS_ATTACH_METHOD.

Fixes LP Bug #1131768.

Change-Id: I7e9f010022731a43e8d793cf8415ec12b346fdb4
Merge "Make guestfs use same libvirt URI as Nova."
Merge "Removes unnecessary qemu-img dependency on powervm driver"