lib: rename qemuImg to qemuimg for pep8 compliancy

Modules should have short, all-lowercase names.

Change-Id: I4fad13d1b9b44f93b5fd0532e4d3ed6b55170eb5
Signed-off-by: Federico Simoncelli <fsimonce@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/26866
Reviewed-by: Dan Kenigsberg <danken@redhat.com>
Reviewed-by: Douglas Schilling Landgraf <dougsland@redhat.com>