xen, blkfront: remove uninitialized var warning

This commit addresses a compile-time warning due to a variable
apparently being uninitialized; actually the variable is initialized
in a helper function, so this commit just uses the uninitialized_var()
macro to remove the warning.
xen, blkfront: add module parameter to force the use of blk-mq

This commit adds a module parameter that allows to force the use
of the blk-mq API. The parameter is set to 1 by default; when
used, it allows the driver to use the API with just one hardware
queue and having a non-mq-capable backend.