xen/vpci: Move ecam access functions to common code

xen/vpci: Move ecam access functions to common code

PCI standard is using ECAM and not MCFG which is coming from ACPI[1].
Use ECAM/ecam instead of MCFG in common code and in new functions added
in common vpci code by this patch.

Move vpci_access_allowed from arch/x86/hvm/io.c to drivers/vpci/vpci.c.

Create vpci_ecam_{read,write} in drivers/vpci/vpci.c that
contains the common code to perform these operations, changed
vpci_mmcfg_{read,write} accordingly to make use of these functions.

The vpci_ecam_{read,write} functions are returning false on error and
true on success. As the x86 code was previously always returning
X86EMUL_OKAY the return code is ignored. A comment has been added in
the code to show that this is intentional.

Those functions will be used in a following patch inside by arm vpci
implementation.

Rename MMCFG_BDF to VPCI_ECAM_BDF and move it to vpci.h.
This macro is only used by functions calling vpci_ecam helpers.

No functional change intended with this patch.

[1] https://wiki.osdev.org/PCI_Express

Suggested-by: Roger Pau Monné <roger.pau@citrix.com>
Signed-off-by: Bertrand Marquis <bertrand.marquis@arm.com>
Acked-by: Roger Pau Monné <roger.pau@citrix.com>
Reviewed-by: Stefano Stabellini <sstabellini@kernel.org>