virt.qemu_vm: Add more info to ImageUnbootableError exception

This exception might make user confused, so add some more info
to its message.

Signed-off-by: Qingtang Zhou <qzhou@redhat.com>
tests.cfg: Limit save_restore test on Linux guest

The save_restore.py script contains some linux-specific
operation, thus add a filter to limit it on Linux guest.

Signed-off-by: Qingtang Zhou <qzhou@redhat.com>
qemu.tests.physical_resource_check: Update memory check pattern

Signed-off-by: Qingtang Zhou <qzhou@redhat.com>