make-flight: guest should use jessie to test pvgrub

make-flight: guest should use jessie to test pvgrub

stretch has 64bit feature enabled for ext4, which pvgrub can't cope.
We want to continue to test pvgrub, so specify jessie in the guest
suite field.

A consequence is that this test will test jessie forever.  Eventually
jessie will rot so badly that this test fails and then we will no
longer be testing pvgrub1.  Hopefully by then no-one will be using it.

CC: Juergen Gross <jgross@suse.com>
Signed-off-by: Wei Liu <wei.liu2@citrix.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>
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v4: Adjust commit message slightly.