v2v: Work around buggy `virsh dumpxml'.

The `virsh dumpxml' command doesn't work properly when the libvirt
source requires authentication.  This is because the authentication
prompts are sent to stdout, but stdout is also the place where we are
reading the output XML from.

Add a mini-binding to libvirt virDomainGetXMLDesc which avoids this,
getting the XML directly using the libvirt API.  Change existing
external calls to `virsh dumpxml' to use this API instead.
v2v: Implement binding for `virsh pool-dumpxml'.

Add a mini-binding to libvirt virStoragePoolGetXMLDesc to get the pool
XML directly using the libvirt API.  Change existing external calls to
`virsh pool-dumpxml' to use this API instead.
v2v: Remove bogus %systemroot%\Drivers path.

This was based an incorrect translation of what the old virt-v2v was
doing.  %systemroot%\Drivers is not expected to exist already, and
indeed does not exist in a real Windows 7 guest.
v2v: utils: Don't need to use module name (Common_utils.) here.
v2v: utils: Permit more safe/unquoted characters in url_quote function.