Improve some errors for openconsole/channel

Functions like virDomainOpenConsole() and virDomainOpenChannel() accept
NULL as a dev_name parameter.  Try using alias for the error message if
dev_name is not specified.

Before:
  error: internal error: character device <null> is not using a PTY

After:
  error: internal error: character device serial0 is not using a PTY
Signed-off-by: NLuyao Huang <lhuang@redhat.com>

src/libxl/libxl_driver.c

@@ -4299,7 +4299,7 @@ libxlDomainOpenConsole(virDomainPtr dom,
     if (chr->source.type != VIR_DOMAIN_CHR_TYPE_PTY) {
         virReportError(VIR_ERR_INTERNAL_ERROR,
                        _("character device %s is not using a PTY"),
-                       NULLSTR(dev_name));
+                       dev_name ? dev_name : NULLSTR(chr->info.alias));
         goto cleanup;
     }
 

src/lxc/lxc_driver.c

@@ -3484,7 +3484,8 @@ lxcDomainOpenConsole(virDomainPtr dom,
 
     if (chr->source.type != VIR_DOMAIN_CHR_TYPE_PTY) {
         virReportError(VIR_ERR_INTERNAL_ERROR,
-                       _("character device %s is not using a PTY"), dev_name);
+                       _("character device %s is not using a PTY"),
+                       dev_name ? dev_name : NULLSTR(chr->info.alias));
         goto cleanup;
     }
 

src/qemu/qemu_driver.c

@@ -15958,7 +15958,7 @@ qemuDomainOpenConsole(virDomainPtr dom,
     if (chr->source.type != VIR_DOMAIN_CHR_TYPE_PTY) {
         virReportError(VIR_ERR_INTERNAL_ERROR,
                        _("character device %s is not using a PTY"),
-                       NULLSTR(dev_name));
+                       dev_name ? dev_name : NULLSTR(chr->info.alias));
         goto cleanup;
     }
 
@@ -16032,7 +16032,7 @@ qemuDomainOpenChannel(virDomainPtr dom,
     if (chr->source.type != VIR_DOMAIN_CHR_TYPE_UNIX) {
         virReportError(VIR_ERR_INTERNAL_ERROR,
                        _("channel %s is not using a UNIX socket"),
-                       NULLSTR(name));
+                       name ? name : NULLSTR(chr->info.alias));
         goto cleanup;
     }
 

src/uml/uml_driver.c

@@ -2640,7 +2640,8 @@ umlDomainOpenConsole(virDomainPtr dom,
 
     if (chr->source.type != VIR_DOMAIN_CHR_TYPE_PTY) {
         virReportError(VIR_ERR_INTERNAL_ERROR,
-                        _("character device %s is not using a PTY"), dev_name);
+                        _("character device %s is not using a PTY"),
+                       dev_name ? dev_name : NULLSTR(chr->info.alias));
         goto cleanup;
     }
 

src/xen/xen_driver.c

@@ -2705,7 +2705,8 @@ xenUnifiedDomainOpenConsole(virDomainPtr dom,
 
     if (chr->source.type != VIR_DOMAIN_CHR_TYPE_PTY) {
         virReportError(VIR_ERR_INTERNAL_ERROR,
-                       _("character device %s is not using a PTY"), dev_name);
+                       _("character device %s is not using a PTY"),
+                       dev_name ? dev_name : NULLSTR(chr->info.alias));
         goto cleanup;
     }