VT-d: fix assertion fault in pci passthrough code

VT-d: fix assertion fault in pci passthrough code

Remove ASSERT(spin_is_locked(&pcidevs_lock)) in
pci_get_pdev_by_domain() to allow caller the flexibility to not hold
the lock if it does not care if the device is hot removed between the
time it got the pdev and the time it is used to get the corresponding
vt-d engine.  In the new RHSA use case, we just wanted to get the vt-d
engine of any device passthrough in the domain. Also, rename RHSA
field "domain" to "proximity domain" to avoid overloading the term
"domain" in virtualization context.

Signed-off-by: Allen Kay <allen.m.kay@intel.com>