xen/pciback: Don't disable PCI_COMMAND on PCI device reset.

There is no need for this at all. Worst it means that if
the guest tries to write to BARs it could lead (on certain
platforms) to PCI SERR errors.

Please note that with af6fc858
"xen-pciback: limit guest control of command register"
a guest is still allowed to enable those control bits (safely), but
is not allowed to disable them and that therefore a well behaved
frontend which enables things before using them will still
function correctly.

This is done via an write to the configuration register 0x4 which
triggers on the backend side:
command_write
  \- pci_enable_device
     \- pci_enable_device_flags
        \- do_pci_enable_device
           \- pcibios_enable_device
              \-pci_enable_resourcess
                [which enables the PCI_COMMAND_MEMORY|PCI_COMMAND_IO]

However guests (and drivers) which don't do this could cause
problems, including the security issues which XSA-120 sought
to address.
Reported-by: Jan Beulich <jbeulich@suse.com>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Reviewed-by: Prarit Bhargava <prarit@redhat.com>
Signed-off-by: Juergen Gross <jgross@suse.com>

drivers/xen/xen-pciback/pciback_ops.c

@@ -127,8 +127,6 @@ void xen_pcibk_reset_device(struct pci_dev *dev)
 		if (pci_is_enabled(dev))
 			pci_disable_device(dev);
 
-		pci_write_config_word(dev, PCI_COMMAND, 0);
-
 		dev->is_busmaster = 0;
 	} else {
 		pci_read_config_word(dev, PCI_COMMAND, &cmd);