tests: test-qga, loosen assumptions about host filesystems

QGA skips pseudo-filesystems when querying filesystems via
guest-get-fsinfo. On some hosts, such as travis-ci which uses
containers with simfs filesystems, QGA might not report *any*
filesystems. Our test case assumes there would be at least one,
leading to false error messages in these situations.

Instead, sanity-check values iff we get at least one filesystem.

Cc: Marc-André Lureau <marcandre.lureau@redhat.com>
Cc: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>
