configure: qemu-ga: explicitly enable qemu-ga MSI support when probed

Currently, if we don't explicitly disable support for MSI installer
via --disable-guest-agent-msi, the configure variable that tracks
the flag, 'guest_agent_msi', never gets set unless one of the probes
fails. Subsequent code then treats this unset value the same as if it
were a "yes" value (via != "no" style checks).

Instead, set the default "yes" value explicitly after the probes, then
make subsequent code expect the values to be set.

This makes it easier to report on whether or not MSI support was
enabled via probe by looking at the ./configure summary.
Signed-off-by: NMichael Roth <mdroth@linux.vnet.ibm.com>

configure

@@ -4378,9 +4378,15 @@ elif ! has wixl; then
     error_exit "MSI guest agent package requires wixl tool installed ( usually from msitools package )"
   fi
   guest_agent_msi=no
+else
+  # we support qemu-ga, mingw32, and wixl: default to MSI enabled if it wasn't
+  # disabled explicitly
+  if test "$guest_agent_msi" != "no"; then
+    guest_agent_msi=yes
+  fi
 fi
 
-if test "$guest_agent_msi" != "no"; then
+if test "$guest_agent_msi" = "yes"; then
   if test "$guest_agent_with_vss" = "yes"; then
     QEMU_GA_MSI_WITH_VSS="-D InstallVss"
   fi
@@ -4659,7 +4665,7 @@ if test "$mingw32" = "yes" ; then
   if test "$guest_agent_ntddscsi" = "yes" ; then
     echo "CONFIG_QGA_NTDDDISK=y" >> $config_host_mak
   fi
-  if test "$guest_agent_msi" != "no"; then
+  if test "$guest_agent_msi" = "yes"; then
     echo "QEMU_GA_MSI_ENABLED=yes" >> $config_host_mak  
     echo "QEMU_GA_MSI_MINGW_DLL_PATH=${QEMU_GA_MSI_MINGW_DLL_PATH}" >> $config_host_mak
     echo "QEMU_GA_MSI_WITH_VSS=${QEMU_GA_MSI_WITH_VSS}" >> $config_host_mak