pci: remove hard-coded bar size in msix_init_exclusive_bar()

This patch lets msix_init_exclusive_bar() can calculate the bar and
pba size based on the number of MSI-X vectors other than using a
hard-coded limit 4096. This is needed to allow device to have more
than 128 MSI_X vectors. To keep migration compatibility, keep using
4096 as bar size and 2048 for pba offset.

Notes: We don't care about the case that using vectors > 128 for
legacy machine type. Since we limit the queue max to 64, so vectors >=
65 is meaningless.

Virtio device will be the first user for this.

Cc: Keith Busch <keith.busch@intel.com>
Cc: Kevin Wolf <kwolf@redhat.com>
Cc: Stefan Hajnoczi <stefanha@redhat.com>
Cc: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Jason Wang <jasowang@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
