PCI: hv: Enable PCI pass-thru devices in Confidential VMs

For PCI pass-thru devices in a Confidential VM, Hyper-V requires
that PCI config space be accessed via hypercalls.  In normal VMs,
config space accesses are trapped to the Hyper-V host and emulated.
But in a confidential VM, the host can't access guest memory to
decode the instruction for emulation, so an explicit hypercall must
be used.

Add functions to make the new MMIO read and MMIO write hypercalls.
Update the PCI config space access functions to use the hypercalls
when such use is indicated by Hyper-V flags.  Also, set the flag to
allow the Hyper-V PCI driver to be loaded and used in a Confidential
VM (a.k.a., "Isolation VM").  The driver has previously been hardened
against a malicious Hyper-V host[1].

[1] https://lore.kernel.org/all/[email protected]/

Co-developed-by: Dexuan Cui <[email protected]>
Signed-off-by: Dexuan Cui <[email protected]>
Signed-off-by: Michael Kelley <[email protected]>
Reviewed-by: Boqun Feng <[email protected]>
Reviewed-by: Haiyang Zhang <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Wei Liu <[email protected]>

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