drivers/virt: Drop VIRT_DRIVERS build dependency

All of the objects in drivers/virt/ have their own configuration symbols to
gate compilation. I.e. nothing gets added to the kernel with
CONFIG_VIRT_DRIVERS=y in isolation.

Unconditionally descend into drivers/virt/ so that consumers do not need to
add an additional CONFIG_VIRT_DRIVERS dependency.

Fix warnings of the form:

Kconfig warnings: (for reference only)
WARNING: unmet direct dependencies detected for TSM
Depends on [n]: VIRT_DRIVERS [=n]
Selected by [y]:
- PCI_TSM [=y] && PCI [=y]

...where PCI_TSM selects CONFIG_TSM, but fails to select
CONFIG_VIRT_DRIVERS.

Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202511041832.ylcgIiqN-lkp@intel.com/
Reviewed-by: Jonathan Cameron <jonathan.cameron@huawei.com>
Link: https://patch.msgid.link/20251113021446.436830-2-dan.j.williams@intel.com
Signed-off-by: Dan Williams <dan.j.williams@intel.com>