Add MSI-X support to pcitest tool.

Modify pcitest.sh script to accommodate MSI-X interrupt tests.

Update documentation accordingly.
Signed-off-by: Gustavo Pimentel <gustavo.pimentel@synopsys.com>
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Acked-by: Kishon Vijay Abraham I <kishon@ti.com>