p2v: Chomp kernel command line (RHBZ#1229340).

When using the kernel command line to configure virt-p2v, if any p2v.*
parameter appeared right at the end of the kernel command line, then a
bogus \n would be appended.  Remove this.

This is essentially the same fix as:
4819655b3c

p2v/main.c

@@ -534,6 +534,9 @@ read_cmdline (void)
     return NULL;
   }
 
+  if (len >= 1 && ret[len-1] == '\n')
+    ret[len-1] = '\0';
+
   return ret;
 }