virt: Fix Kconfig warning when selecting TSM without VIRT_DRIVERS

After commit 3225f52cde56 ("PCI/TSM: Establish Secure Sessions and Link
Encryption"), there is a Kconfig warning when selecting CONFIG_TSM
without CONFIG_VIRT_DRIVERS:

WARNING: unmet direct dependencies detected for TSM
Depends on [n]: VIRT_DRIVERS [=n]
Selected by [y]:
- PCI_TSM [=y] && PCI [=y]

CONFIG_TSM is defined in drivers/virt/coco/Kconfig but this Kconfig is
only sourced when CONFIG_VIRT_DRIVERS is enabled. Since this symbol is
hidden with no dependencies, it should be available without a symbol
that just enables a menu.

Move the sourcing of drivers/virt/coco/Kconfig outside of
CONFIG_VIRT_DRIVERS and wrap the other source statements in
drivers/virt/coco/Kconfig with CONFIG_VIRT_DRIVERS to ensure users do
not get any additional prompts while ensuring CONFIG_TSM is always
available to select. This complements commit 110c155e8a68 ("drivers/virt:
Drop VIRT_DRIVERS build dependency"), which addressed the build issue
that this Kconfig warning was pointing out.

Fixes: 3225f52cde56 ("PCI/TSM: Establish Secure Sessions and Link Encryption")
Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202511140712.NubhamPy-lkp@intel.com/
Signed-off-by: Nathan Chancellor <nathan@kernel.org>
Link: https://patch.msgid.link/20251203-fix-pci-tsm-select-tsm-warning-v1-1-c3959c1cb110@kernel.org
Signed-off-by: Dan Williams <dan.j.williams@intel.com>