Introduce <driver> under <filesystem> to support open-by-handle

Introduce <driver> under <filesystem> to support open-by-handle

VirtFS allows the user to choose between path/handle based fs driver.
As of now, libvirt hardcoded path based driver only. This patch provides
a solution to allow user to choose between path/handle based fs driver.

Sample:

    <filesystem type='mount'>
      <driver type='handle'/>
      <source dir='/folder/to/share1'/>
      <target dir='mount_tag1'/>
    </filesystem>

    <filesystem type='mount'>
      <driver type='path'/>
      <source dir='/folder/to/share2'/>
      <target dir='mount_tag2'/>
    </filesystem>

Signed-off-by: Harsh Prateek Bora <harsh@linux.vnet.ibm.com>
Signed-off-by: Eric Blake <eblake@redhat.com>