tests: Disable dbus-vmstate-test

The dbus-vmstate-test has been failing in some Patchew configs
since about the 6th March:

  dbus-daemon[9321]: Could not get password database information for UID of current process: User "???" unknown or no memory to allocate password entry

  **
  ERROR:/tmp/qemu-test/src/tests/qtest/dbus-vmstate-test.c:114:get_connection: assertion failed (err == NULL): The connection is closed (g-io-error-quark, 18)
  cleaning up pid 9321
  ERROR - Bail out! ERROR:/tmp/qemu-test/src/tests/qtest/dbus-vmstate-test.c:114:get_connection: assertion failed (err == NULL): The connection is closed (g-io-error-quark, 18)
  make: *** [/tmp/qemu-test/src/tests/Makefile.include:632: check-qtest-x86_64] Error 1
  make: *** Waiting for unfinished jobs....

It's not clear why this is happening (perhaps a recently revealed
race condition or a change in the patchew build environment?).

For the moment, disable this test so that patchew test runs are
useful and don't email the list with spurious failure mails.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Message-id: 20200310152141.13959-1-peter.maydell@linaro.org