x86: pcie: fix allocating 1 vector under MSI-X

Fix an issue where 1 vector is being requested when MSI-X is
enabled. The previous logic always assumed the PCIE device has
only fixed or single MSI when we are requesting 1 vector, which
is not entirely correct. So if there is no vector allocated
already, try to allocate one.

Fixes #93319

Signed-off-by: Daniel Leung <daniel.leung@intel.com>