block: char devices on FreeBSD are not behind a pager

block: char devices on FreeBSD are not behind a pager

Introduce a new flag to mark devices that require requests to be aligned and
replace the usage of BDRV_O_NOCACHE and O_DIRECT with this flag when
appropriate.

If a character device is used as a backend on a FreeBSD host set this flag
unconditionally.

upstream-commit-id: 3cad83075c7b847fe0eb6e61316fdf50984d4570

Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Reviewed-by: Max Reitz <mreitz@redhat.com>
Cc: Kevin Wolf <kwolf@redhat.com>
Cc: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Max Reitz <mreitz@redhat.com>
Signed-off-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>