In a number of APIs, the text implied that a user might have
<target dev='xvda'/> - but common convention is to use "vda",
not "xvda".  For example, virDomainGetDiskErrors was correct,
while virDomainBlockStats was confusing.

* src/libvirt.c: Make examples consistent.
Signed-off-by: NEric Blake <eblake@redhat.com>