Rename msix_supported to msi_supported and control MSI and MSI-X
activation this way. That was likely to original intention for this
flag, but MSI support came after MSI-X.
Signed-off-by: NJan Kiszka <jan.kiszka@siemens.com>

implements msi related functions.
Signed-off-by: NIsaku Yamahata <yamahata@valinux.co.jp>
Signed-off-by: NMichael S. Tsirkin <mst@redhat.com>