[Libguestfs] [PATCH] tests: Increase appliance memory when testing 256+ disks.

Currently the tests fail on x86 with recent kernels: FAIL: test-255-disks.sh This confused me for a while because our other test program (utils/max-disks/max-disks.pl) reports that it should be possible to add 255 disks. Well it turns out that the default amount of appliance memory is sufficient if you're just adding disks, but if you try to add _and_ partition those disks there's insufficient memory and the daemon falls over with an out of memory error. I considered increasing the default appliance memory, and this is certainly one way to fix the problem. However this penalises every user for what is a fairly niche use case. This takes an alternative approach of increasing the appliance memory for the affected tests. Related links: https://bugzilla.redhat.com/show_bug.cgi?id=1478201 https://rwmj.wordpress.com/2017/04/28/how-many-disks-can-you-add-to-a-vir... Rich.