Stop free'ing 'const char *' strings

Stop free'ing 'const char *' strings

The VIR_FREE() macro will cast away any const-ness. This masked a
number of places where we passed a 'const char *' string to
VIR_FREE. Fortunately in all of these cases, the variable was not
in fact const data, but a heap allocated string. Fix all the
variable declarations to reflect this.

Signed-off-by: Daniel P. Berrange <berrange@redhat.com>