passthrough: Fix older kernel assigned device resume failure problem

passthrough: Fix older kernel assigned device resume failure problem

We found when do S3 resume, some older kernel(such as rh5.1)
cmd register might be restored before BAR registers, Since
pt_bar_mapping is done when writing cmd register, it will cause
mapping incorrect address.
It's the root_cause of when doing RH5.1 HVM S3, assigned e1000e
network card fails to resume back. (set pci_power_mgmt=1 in guest
config file, let PM support device enter D3 hot state during guest S3)

we should call pt_bar_mapping in pt_bar_reg_write to avoid this problem
for supporting older kernels.

Signed-off-by: Liping Ke <Liping.Ke@intel.com>
Signed-off-by: Yu Zhao<yu.zhao@intel.com>
Acked-by: Yuji Shimada <shimada-yxb@necst.nec.co.jp>