block/xen-blkfront: Split blkif_queue_request in 2

block/xen-blkfront: Split blkif_queue_request in 2

Currently, blkif_queue_request has 2 distinct execution path:
    - Send a discard request
    - Send a read/write request

The function is also allocating grants to use for generating the
request. Although, this is only used for read/write request.

Rather than having a function with 2 distinct execution path, separate
the function in 2. This will also remove one level of tabulation.

Signed-off-by: Julien Grall <julien.grall@citrix.com>
Reviewed-by: Roger Pau Monné <roger.pau@citrix.com>
Signed-off-by: David Vrabel <david.vrabel@citrix.com>
(cherry picked from commit 33204663ef85d9ce79009b2246afe6a2fef8eb1b)
Signed-off-by: Joao Martins <joao.m.martins@oracle.com>