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: Michael Roth <mdroth@linux.vnet.ibm.com>
