virtiofsd: Move core file creation code in separate function

Move core file creation bits in a separate function. Soon this is going
to get more complex as file creation need to set security context also.
And there will be multiple modes of file creation in next patch.

Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Signed-off-by: Vivek Goyal <vgoyal@redhat.com>
Message-Id: <20220208204813.682906-7-vgoyal@redhat.com>
Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>