msix: make pba size math more uniform

msix: make pba size math more uniform

In msix_exclusive_bar the bar_pba_size is more than what the pba is
expected to have, although this never affects the bar size.

Specifically, the math in msix_init_exclusive_bar allocates too much
memory in some cases.

For example consider nentries = 8.  msix_exclusive_bar will give us
bar_pba_size = 16.  So 16 bytes.  However 8 bytes would be enough - this
is all that the spec requires.

So in practice bar_pba_size sometimes allocates an extra 8 bytes but
never more.

Since each MSIX entry size is 16 bytes, and since we make sure that
table+pba is a power of two, this always leaves a multiple of 16 bytes
for the PBA, so extra 8 bytes have no effect.

However, its ugly to have pba size temporary variable have an incorrect
value.  For consistency switch to the formula used in msix_init.

Signed-off-by: Dongli Zhang <dongli.zhang@oracle.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>