pci: Implement callbacks for toggling BAR emulation

Implement callbacks for activating and deactivating emulation for a BAR
region. This is in preparation for allowing a guest operating system to
enable and disable access to I/O or memory space, or to reassign the
BARs.

The emulated vesa device framebuffer isn't designed to allow stopping and
restarting at arbitrary points in the guest execution. Furthermore, on x86,
the kernel will not change the BAR addresses, which on bare metal are
programmed by the firmware, so take the easy way out and refuse to
activate/deactivate emulation for the BAR regions. We also take this
opportunity to make the vesa emulation code more consistent by moving all
static variable definitions in one place, at the top of the file.

Signed-off-by: Alexandru Elisei <alexandru.elisei@arm.com>
Reviewed-by: Andre Przywara <andre.przywara@arm.com>
Link: https://lore.kernel.org/r/1589470709-4104-9-git-send-email-alexandru.elisei@arm.com


Signed-off-by: Will Deacon <will@kernel.org>