aer: impove pcie_aer_init to support vfio device

aer: impove pcie_aer_init to support vfio device

pcie_aer_init was used to emulate an aer capability for pcie device,
but for vfio device, the aer config space size is mutable and is not
always equal to PCI_ERR_SIZEOF(0x48). it depends on where the TLP Prefix
register required, so here we add a size argument.

Signed-off-by: Chen Fan <chen.fan.fnst@cn.fujitsu.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Alex Williamson <alex.williamson@redhat.com>