'msi' target reports error if we attempt to use it when QEMU hasn't
been ./configure'd to enable it. The parenthesis cause an interpreter
error if we don't enclose the error in quotes.

Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>