conf: Display <physical> in output of voldef

conf: Display <physical> in output of voldef

Although the virStorageBackendUpdateVolTargetInfo will update the
target.physical value, there is no way to provide that information
via the virStorageGetVolInfo API since it only returns the capacity
and allocation of a volume. So as described in commit id '0282ca45',
it should be possible to generate an output only <physical> value
for that purpose.

This patch generates the <physical> value in the volume XML output
for the sole purpose of being able to view/see the value to allow
someone to parse the XML in order to obtain the value.

Update the documentation to describe the output only nature.

Signed-off-by: John Ferlan <jferlan@redhat.com>