We piggyback on "xen/swiotlb: Use the swiotlb_late_init_with_tbl to init
Xen-SWIOTLB late when PV PCI is used." functionality to start up
the Xen-SWIOTLB if we are hot-plugged. This allows us to bypass
the need to supply 'iommu=soft' on the Linux command line (mostly).
With this patch, if a user forgot 'iommu=soft' on the command line,
and hotplug a PCI device they will get:

pcifront pci-0: Installing PCI frontend
Warning: only able to allocate 4 MB for software IO TLB
software IO TLB [mem 0x2a000000-0x2a3fffff] (4MB) mapped at [ffff88002a000000-ffff88002a3fffff]
pcifront pci-0: Creating PCI Frontend Bus 0000:00
pcifront pci-0: PCI host bridge to bus 0000:00
pci_bus 0000:00: root bus resource [io  0x0000-0xffff]
pci_bus 0000:00: root bus resource [mem 0x00000000-0xfffffffff]
pci 0000:00:00.0: [8086:10d3] type 00 class 0x020000
pci 0000:00:00.0: reg 10: [mem 0xfe5c0000-0xfe5dffff]
pci 0000:00:00.0: reg 14: [mem 0xfe500000-0xfe57ffff]
pci 0000:00:00.0: reg 18: [io  0xe000-0xe01f]
pci 0000:00:00.0: reg 1c: [mem 0xfe5e0000-0xfe5e3fff]
pcifront pci-0: claiming resource 0000:00:00.0/0
pcifront pci-0: claiming resource 0000:00:00.0/1
pcifront pci-0: claiming resource 0000:00:00.0/2
pcifront pci-0: claiming resource 0000:00:00.0/3
e1000e: Intel(R) PRO/1000 Network Driver - 2.0.0-k
e1000e: Copyright(c) 1999 - 2012 Intel Corporation.
e1000e 0000:00:00.0: Disabling ASPM L0s L1
e1000e 0000:00:00.0: enabling device (0000 -> 0002)
e1000e 0000:00:00.0: Xen PCI mapped GSI16 to IRQ34
e1000e 0000:00:00.0: (unregistered net_device): Interrupt Throttling Rate (ints/sec) set to dynamic conservative mode
e1000e 0000:00:00.0: eth0: (PCI Express:2.5GT/s:Width x1) 00:1b:21:ab:c6:13
e1000e 0000:00:00.0: eth0: Intel(R) PRO/1000 Network Connection
e1000e 0000:00:00.0: eth0: MAC: 3, PHY: 8, PBA No: E46981-005

The "Warning only" will go away if one supplies 'iommu=soft' instead
as we have a higher chance of being able to allocate large swaths of
memory.
Acked-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>