In preparation libtool "-module" flag removal, add lib prefix to all
mock shared objects.

While at it, introduce VIR_TEST_MOCK macros that makes path out of mock
name to be used with VIR_TEST_PRELOAD or VIR_TEST_MAIN_PRELOAD.  That,
hopefully, improves readability, reduces line length and allows to
tailor VIR_TEST_MOCK for specific platform if it has shared library
suffix different from ".so".
Signed-off-by: NRoman Bolshakov <r.bolshakov@yadro.com>