pci: Add generic ECAM host support

Unlike x86, other architectures using generic ECAM PCI host
do not have a luxury of PCI bus initialized by a BIOS and
ready to use at start. Thus, we need allocate and assign
resources to all devices, much like an architecture's
firmware would do.

There is no any sort of resource management for memory and
io spaces, since only ones-per-BAR allocations are expected
and no deallocations at all.

Cc: Thomas Huth <thuth@redhat.com>
Cc: Andrew Jones <drjones@redhat.com>
Cc: Peter Xu <peterx@redhat.com>
Suggested-by: Andrew Jones <drjones@redhat.com>
Signed-off-by: Alexander Gordeev <agordeev@redhat.com>
Message-Id: <6488b060e4cbbead5a5ec525a53d65773ceb5f87.1478512824.git.agordeev@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>