oeqa/sve: Add DOMU_INSTANCES check for domu2

This commit adds a new check for the DOMU_INSTANCES before
proceeding with the domu2 test cases. The purpose of this check
is to ensure that domu2 instances available before running the
test case. If only one instance is available, the test case for
domu2 will be skipped.

Signed-off-by: Divin Raj <divin.raj@arm.com>