virBitmapParse: Fix behavior in case of error and fix up callers

Re-arrange the code so that the returned bitmap is always initialized to
NULL even on early failures and return an error message as some callers
are already expecting it. Fix up the rest not to shadow the error.

tests/cpuset

@@ -42,7 +42,7 @@ sed "s/vcpu placement='static'>/vcpu cpuset='aaa'>/" xml > xml-invalid || fail=1
 $abs_top_builddir/tools/virsh --connect test:///default define xml-invalid > out 2>&1 && fail=1
 cat <<\EOF > exp || fail=1
 error: Failed to define domain from xml-invalid
-error: XML error: topology cpuset syntax error
+error: invalid argument: Failed to parse bitmap 'aaa'
 
 EOF
 compare exp out || fail=1